NAIROBI, Kenya — A policeman gunned down an opposition lawmaker Thursday in an attack officials described as a crime of passion. Protesters burned houses in revenge, convinced it was an assassination in a nation gripped by ethnic fighting since a disputed election.

Police said David Too was shot by a police officer who discovered the lawmaker was having an affair with his girlfriend. A woman shot in the same attack also died, a hospital official said.

Too was the second anti-government legislator killed in a week; opposition politicians said both were victims of assassination plots. A family spokesman accused the police of a cover-up, saying the lawmaker was not involved with the woman and had feared for his safety.

"Too had expressed fears that his life was in danger, especially during the campaign period as he crisscrossed the constituency to seek votes," said Julius Langat.

Thousands of people from Too's Kalenjin ethnic group sought revenge by setting houses on fire and blocking roads with rocks on the outskirts of the western town of Kericho, near the slain lawmaker's constituency. Other tribes took refuge at the police station. Similar violence broke out in other towns.

Kenya has been embroiled in ethnic violence since President Mwai Kibaki was declared the winner of the Dec. 27 election and opposition candidate Raila Odinga rejected the result, saying the vote had been rigged.

Much of the bloodshed has pitted other tribes, including Odinga's Luo, against Kibaki's Kikuyu people. Kikuyus, Kenya's largest ethnic group, have long been resented for their dominance of the economy and politics. Western Kenya's Rift Valley has seen some of the worst violence.

Odinga has said he wants a new election, while Kibaki has made clear he will not negotiate his position as president. In Nairobi, negotiators from the two camps began the first day of talks mediated by former U.N. Secretary-General Kofi Annan.

At a summit in Ethiopia, the head of the African Union warned leaders of the continent they could not stand by while Kenya descended into ethnic cleansing and possibly genocide.

Too's death came two days after another opposition lawmaker, Mugabe Were, was fatally shot as he drove to his house in suburban Nairobi, setting off more violence in the capital's slums and in western regions.

Opposition party secretary-general Anyang Nyongo said there was "an evil scheme" to kill legislators and rob the opposition of its majority in parliament. Legislative elections held the same day as the presidential vote gave the opposition 99 parliament seats to 43 for Kibaki's party.

Were and Too would have been freshmen lawmakers. While neither was seen as particularly influential, they had local followings that defeated incumbents in hard-fought races.

Henry Kosgie, an official of the opposition Orange Democratic Movement, told reporters that witnesses had reported seeing Too shot as he traveled by car from Nairobi to the western city of Eldoret. An Associated Press reporter saw Too's body at a hospital in Eldoret.

But police said Too's death was not linked to the political turmoil. Eldoret Deputy Police Chief Gabriel Kuya said the traffic officer had discovered that his girlfriend was having an affair with Too, and chased the pair on his motorcycle when he saw them together in a car.

"He drove toward the side of the woman and shot her in the stomach twice. Her partner (legislator Too) pleaded with the officer not to kill her but he turned his pistol on him instead, hitting him four times in the head," Kuya told the AP.

Langat, the Too family spokesman, said the woman was a police officer and that Too had gone to her to seek protection for his family.

"I know the woman who was with Too very well," Langat said, speaking from Too's home village of Chepkoiyo. "In fact she is my relative and there was no way that she was involved with the legislator in a love triangle."

Within minutes of the news reaching the western city of Kisumu, an opposition stronghold, gangs of men armed themselves with machetes and set up burning barricades. Businesses shut down and workers fled the town center.

The sky turned black with smoke from burning tires. A truck was set on fire outside the police station, and a mob lobbed stones at the station.

"Kibaki must go! They are finishing off our leaders!" the protesters yelled.

Police and protesters lobbing stones at them engaged in running battles, then police fired into the crowd as a reporter watched. He saw a middle-aged man, shot in the stomach, lying in the street.

A protester, Jane Akelo, said she saw an officer shoot a teenager in the head and the youth died on the spot. She said she saw another body lying on the road.

"We won't believe what the government says. We know they are involved in the killings," said Wilis Omondi, a laboratory technician in Kisumu.

The top U.S. diplomat for Africa, Assistant Secretary of State for African Affairs Jendayi Frazer, said Wednesday that she saw the violence as ethnic cleansing. But the State Department backed away from her statement, saying the U.S. had not yet concluded whether atrocities had been committed.

Hundreds of Kikuyus have been killed, and members of the group account for more than half of the 300,000 chased from their homes, most in the Rift Valley.

Human rights groups and others accuse politicians of orchestrating some of the violence.

At the African Union summit, AU chairman Alpha Konare said Africa cannot thrive unless Kenya does, so Africans must make solving its crisis a priority.

"Kenya is a country that was a hope for the continent. Today, if you look at Kenya you see violence on the streets. We are even talking about ethnic cleansing, we are even talking about genocide. We cannot sit with our hands folded."

Kenya's President Kibaki listened from the front row. Odinga had tried to prevent him from attending and appealed to the 52-nation bloc not to recognize him.

Heidi Vogt reported from Addis Ababa, Ethiopia. Associated Press Writers Anita Powell in Addis Ababa, Katy Pownall in Kikuyu Town, Katharine Houreld in Kisumu, and Tom Maliti, Malkhadir M. Muhumed and Tom Odula in Nairobi contributed to this report.