Prime Minister Sirimavo Bandaranaike, who is in poor health and mostly works from home, had been scheduled to go to her office for a day of meetings with visitors, an aide said.
Because of increased security, police spotted the bomber outside the walled, British-built office complex. "Our people found her movement suspicious, and they tried to check her when she exploded herself," said Deputy Inspector General of Police Jagath Jayawardene.
It was the second suicide bombing in the capital in 18 days.
President Chandrika Kumaratunga, Bandaranaike's daughter, suffered a serious eye injury on Dec. 18 when a suicide bomber, also believed to be a Tamil Tiger rebel, blew herself up at an election rally, killing 23 and wounding more than 100.
Mangled bodies and pools of blood covered the street Wednesday as panicked police and soldiers covered the corpses with sheets and shooed away passers-by.
At least four policemen were among the six people immediately killed. Of the 29 people hospitalized, seven died.
Nimal Sirisena, a security guard at the nearby Russian Embassy, said he saw police trying to search the bomber. "She was resisting it, and moments later, I heard a big bang," Sirisena said, wiping away drops of blood that had splattered him.
Hours after the suicide attack, gunmen stopped a pro-rebel politician, Kumar Ponnambalam, in a Tamil neighborhood as he was driving a black Mercedes and fatally shot him in the head. Ponnambalam, 60, had run for president in 1982 and often defended Tamils in human rights cases against the government.
A police official said officers were searching for a man who had been with Ponnambalam, and disappeared after the shooting, along with the victim's mobile phone and wristwatch.
The Liberation Tigers of Tamil Eelam have been fighting for a separate homeland for the minority Tamil people since 1983, in a war that has killed 61,000 people.
The armed forces had succeeded in recent years in pushing the rebels out of major strongholds in northern Jaffna peninsula and into the jungles. The number of bombings in the capital and elsewhere in the Indian Ocean island had diminished.
But during the past few months, the guerrillas, who receive funding, arms, and boats from outside Sri Lanka, have made a strong counterpush. In November, they overran a string of army-held towns, and claimed to have killed hundreds of soldiers.
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