In a country where there is no vocation more perilous, indeed more radical, than being a voice for peace, Neelan Tiruchelvam had worked to bring an end to the savage ethnic conflict that has tormented this lush island nation.

His friends and family had watched in dread as other leaders of his moderate ethnic Tamil political party were gunned down or blown up by guerrillas fighting for a separate homeland for Sri Lanka's Hindu Tamil minority.

When some of them pleaded with Mr. Tiruchelvam, a member of Parliament who helped draft the Government's peace plan, to get out of Sri Lanka with his life, he told them: ''I can't run away. My duty is here.''

So on a hot, sunny morning last month, he went to work as usual, with armed men in a jeep behind him and on a motorcycle in front. And as usual, he got stuck at a busy intersection just a couple of blocks from his whitewashed bungalow.

As he turned the corner, a stranger walked up to his car, pressed his belly against the window and detonated a bomb strapped to his waist.

Mr. Tiruchelvam's shattered body exploded out the other side of his car, and hung limply out the window like a rag doll. The suicide bomber's severed head flew over the vehicle and landed on the curb.

The attack, brazenly carried out just a stone's throw from the Prime Minister's residence, has provided yet more evidence of the brutal effectiveness of the terror tactics used by separatist rebels in silencing fellow Tamils who favor political compromise over armed struggle.

More than any other Tamil leader, it seemed that the Harvard-educated Mr. Tiruchelvam (pronounced teer-oo-CHELL-vum), 55, had the intellectual stature, the gentle temperament and the high-level contacts to coax from the majority Sinhalese rulers constitutional changes to redress the grievances of the Tamil people.

His death has been a blow to the search for a peaceful end to Sri Lanka's 16-year-old civil war, which has left 60,000 dead in a country whose population -- 18 million -- is about the size of New York State's.

Sri Lanka's Justice Minister, G. L. Peiris, a Sinhalese lawyer, said his dead colleague was a bridge between the Tamils and the Sinhalese -- a bridge that the guerrillas wanted to blow up. ''We are now without a consensus builder,'' Mr. Peiris said.

The package of constitutional reforms that Mr. Tiruchelvam and Mr. Peiris crafted to give Tamils more power is still stuck in the poisonous partisan deadlock between the country's two main political parties, both dominated by the Buddhist Sinhalese majority. That deadlock is unlikely to be broken before national elections next year, political experts say.

While the rebels, who call themselves the Liberation Tigers of Tamil Eelam, have neither denied nor claimed responsibility for Mr. Tiruchelvam's slaying, they have been blamed for it by officials and political experts. They note that the group had demonized Mr. Tiruchelvam in their propaganda and that the method of killing -- by a suicide bomber -- is a Tiger signature.

At the end of his life, Mr. Tiruchelvam, a constitutional scholar, was discouraged by the lack of progress in getting the peace package through Parliament. But his wife and colleagues say he never lost hope that a resolution to Sri Lanka's fratricidal war lay in ideas of federalism, individual liberty and tolerance.

''He still felt that it was in his grasp to bring about this historic settlement,'' said Gowher Rizvi, a friend and former professor at Oxford University who now heads the Ford Foundation's South Asia office. ''He would say, 'We're almost there. Our work is almost done.' ''

For Tamil moderates like Mr. Tiruchelvam, 1994 was a year of great hope. They believed that the majority Sinhalese might agree to a just settlement of Tamil grievances and that the guerrillas might accept a peace deal to end the fighting in the country, off India's southern tip.

That year, Chandrika Kumaratunga, daughter of the powerful political family most associated with the rise of Sinhalese nationalism, had won a landslide victory on an unprecedented peace platform.

Her victory came with support from Tamils after she promised to open peace talks with the separatist Tiger militants. Many Tamils believed that her Government would deliver a plan to give them a measure of political autonomy in the north and east of the country, where Tamils are in a majority.

The clash between Tamils and Sinhalese -- over language, religion, university admissions and government patronage, among other things -- had festered since the country gained its independence from the British in 1948.

The Tamils, who make up slightly less than a fifth of the population and who had a privileged position under the colonial rulers, felt discriminated against after the Sinhalese, who are about three-quarters of the population, took the reins of governance.

For Mr. Tiruchelvam, Mrs. Kumaratunga's election seemed to be a moment he had been preparing for his whole life.

He was an upper-caste Hindu born into a politically engaged Tamil family that was part of Colombo's English-speaking elite.

After he earned his doctorate at Harvard Law School in 1972 as a Fulbright scholar, Mr. Tiruchelvam came home and spent his career searching for ways to end Sri Lanka's ethnic strife. He helped organize the International Center for Ethnic Studies, a research group here, and became an expert on how countries had accommodated ethnically diverse societies constitutionally.

He and his wife, Sithie, also a lawyer, built an influential, multi-ethnic circle of friends. She described him as ''a small-made man,'' just 5 foot 3 and a half inches tall, whose shy, quiet ways complimented her own teasing gregariousness.

Roberto Unger, a professor at Harvard Law School who had been close to Mr. Tiruchelvam since their student days at Harvard, said, ''He was completely devoted to reconciliation on the basis of this intuition he had that we have to forgive one another before we can talk to each other.''

During her long years in the political wilderness, the new Sinhalese President, Mrs. Kumaratunga, had joined the Tiruchelvams around the dinner table in their home.

''In political discussions with Neelan, she always felt that the Tamils had gotten a raw deal that had to be corrected,'' Mrs. Tiruchelvam said. ''He believed she had the courage of her convictions and that she would carry out her promises.''

So in 1994, when Mrs. Kumaratunga became President with a huge mandate for peace and Mr. Tiruchelvam was elected to Parliament, he began working with her Government to draft a package of constitutional reforms that were to turn Sri Lanka from a centralized state into an ''indissoluble union of regions.''

As that work progressed, however, negotiations with the guerrillas fell apart in 1995. The Government, in turn, stepped up its military campaign against the Tigers.

Ominously, by late 1995, the authorities had learned that the Tigers planned to stake out Mr. Tiruchelvam's home. The Government began providing him with round-the-clock security. He never again left his walled enclave without bodyguards.

The peace plan stalled. It needed a two-thirds majority in the 225-member Parliament to pass, but opposition from the other major Sinhalese party, the United National Party, left the governing People's Alliance coalition some 17 votes short.

Then in 1997 and 1998, the Tigers, who had already killed a dynamic leader of Mr. Tiruchelvam's party over tea and biscuits in 1989, began killing more of the party's most promising talent, officials say.

Vellupillai Prabhakaran, the underground leader of the Tigers, had built a battle-hardened corps of 4,000 to 8,000 followers, as well an elite cadre of suicide bombers, celebrated as martyrs to Tamil Eelam, the separate country they were fighting for.

First, they gunned down a member of Parliament at a ribbon cutting. Then they killed the newly elected Mayor of the northern city of Jaffna, herself the widow of an assassinated partyman, and a few months later killed her replacement, too.

Mr. Tiruchelvam's elder son, Nirgunan, 26, an investment banker in Singapore, became almost obsessed with his father's security. He begged his father to stay inside their house, or to wear a bullet proof vest and travel in a bomb-proof car.

The son tracked down an aging bomb-proof Jaguar that had carried the Queen of England when she visited Sri Lanka in the early 1980's. But when his father used the car, it broke down. The one garage that could fix it always seemed to be busy.

The signs that Mr. Tiruchelvam was a target mounted. D. B. S. Jeyaraj, a Sri Lankan journalist living in Canada, kept up with the Tigers' propaganda and saw that their verbal attacks on Mr. Tiruchelvam as a traitor to the separatist cause were becoming more venomous.

He cautioned Mr. Tiruchelvam, who replied that there was little he could do to stop the Tigers if they were determined to get him, especially in Colombo, where he was often immobilized in traffic jams.

Carol Grodzins, a longtime family friend who was an administrator at the John F. Kennedy School of Government at Harvard, decided to find a way to get the Tiruchelvam's to safety, if only for a few months.

She phoned Mr. Tiruchelvam's admirers at the Rockefeller Foundation and Harvard Law School. Soon, he had offers from both places and accepted them.

For a month this summer, the Tiruchelvams went to the Rockefeller Foundation's study center in Bellagio, Italy. Mrs. Grodzins had expected them to go directly from Italy to Cambridge, Mass., where Mr. Tiruchelvam was to be a visiting professor at Harvard Law School for the fall semester.

Instead, the couple returned to Colombo on July 19. There was widespread speculation that Mrs. Kumaratunga's Government planned finally to introduce the peace plan in Parliament this August.

The day they got back, the Government warned Mr. Tiruchelvam of a new assassination threat from the Tigers, but did nothing to increase his security. He considered asking the Government for more protection, but decided against it.

''Neelan did not want to be under obligation,'' his wife said. ''He did not want to ask for something that was not given.''

Despite the strain, he carried on with his work. On the morning of July 29, he and his wife had breakfast together as usual.

He had a long telephone chat with his journalist friend in Canada, Mr. Jeyaraj. Mr. Tiruchelvam spoke of the Tigers' latest threats, and said he was convinced that they were out to extinguish his party, the Tamil United Liberation Front. But he needed just a little more time in Parliament to see the constitutional reforms through, he said. Then he would relinquish his seat.

Half an hour later, he got in his car to go to work. At 9:18 A.M. the suicide bomber's blast rocked the neighborhood.

In the weeks since, Mr. Tiruchelvam's fellow Tamil moderates have become even more fearful. Party workers recently filled rusty tar barrels with sand and lined them up at the entrance to the party's shabby headquarters in Colombo to block any bomb-laden car.

One recent afternoon, Mr. Tiruchelvam's sons sat with their mother in their father's darkened, book-lined study and spoke of what the national tragedy has cost them.

After anti-Tamil riots swept Colombo in 1983 and Sinhalese mobs killed hundreds of Tamils, there was an exodus of Tamils from the country. Even then, when virtually every Tamil moderate who had been in Parliament moved to India, Mr. Tiruchelvam refused to leave.

But he and his wife decided to send their sons, then 8 and 10, to boarding school in India to protect them. Mithran, now a 24-year-old law student at Cambridge University, still seems wounded by the experience of growing up so far from his parents.

''It was very hard for the younger one,'' Mrs. Tiruchelvam said, as her sons listened. ''He felt unwanted because he wasn't allowed to come home.''

The sons admire their father's commitment and sacrifice, but agree on this: The family tradition in Sri Lankan politics ends with them. They want no part of it.

Photo: Neelan Tiruchelvam, right in an undated photo, having lunch in a restaurant with his wife, Sithie, and son Mithran. Mr. Tiruchelvam, a Tamil moderate, stayed in Sri Lanka despite rising threats from Tamil militants.

(Universal Photo/Kent Productions)(pg. A8) Map of Sri Lanka highlighting Colombo: A suicide bomber assassinated Mr. Tiruchelvam in Colombo in July.(pg A8)