It was a classic crime of passion, one man taking revenge on a cheating lover. But in Kenya, a country balanced on a knife-edge of tribal and political tensions, it unleashed a fresh wave of violence.

Across an area spanning hundreds of miles in Kenya's west, houses were set ablaze, trucks were torched and gangs of youths armed with bows and arrows hauled drivers from their cars. Three people were shot and injured as security forces fired into the air to disperse them.

The violence erupted when David Too, an opposition MP, was shot dead by a traffic policeman at a road block in the Rift Valley town of Eldoret.

Mr Too was driving from Nairobi to his constituency of Eldoret when the policeman recognised his travelling companion: it was the woman with whom he shared his home.

Colleagues said that the unnamed officer already knew his girlfriend was having an affair with Mr Too and chased them on his motorcycle when he saw them together.

“He drove toward the side of the woman and shot her in the stomach twice,” said Gabriel Kuya, deputy police chief. “Her partner pleaded with the officer not to kill her but he turned his pistol on him instead, hitting him four times in the head.”

In a country where tribe has become a political badge, it did not take long for details of the officer's ethnic grouping to spread around town.

Neighbours said that the killer belonged to the Kisii tribe, accused by Mr Too's Kalenjins of backing President Kibaki, turning a love triangle into a tribal mess. Mr Too supported Raila Odinga, who is engaged in a bitter power struggle with Mr Kibaki.

Appeals for calm were ignored by protesters convinced that it was an assassination. They took to the streets across western Kenya bringing several key towns and cities to a standstill.

Within minutes of the shooting youths armed with machetes were roaming Kisumu, an opposition stronghold on the shores of Lake Victoria. The sky turned black as they burnt tyres and blocked roads.

Shops shut in Eldoret after the killing and some residents began to leave town. “When we heard about the death of the MP everybody came out,” Willy Kiboi, a resident, said. “In a flash, a Kenyan Army Land Rover appeared and they started shooting. One Sudanese man is dead and three people are injured.”

About 200 people gathered outside the police station demanding that the officer responsible be handed over. “Let him be brought out so we can do our own justice,” one protester shouted. “This is a government plot to wipe out ODM [Orange Democratic Movement].”

Thousands of Mr Too's supporters from the Kalenjin tribe blocked roads with rocks and set houses ablaze on the outskirts of the western town of Kericho near his constituency. Other tribes fled to the police station for safety.

Mr Too is the second opposition MP to be shot dead within days. Mugabe Were was killed in apparent robbery as he returned to his Nairobi home this week. Opposition leaders insisted that they were victims of politically motivated assassinations.

“This is part of an evil scheme, to rob the party of its majority in parliament,” said Anyang Nyongo, secretary-general of the ODM.

Trouble has flared repeatedly since Mr Kibaki claimed victory in disputed elections in December. Mr Odinga accuses him of stealing the election in a process that observers have agreed was flawed.

More than 850 people have died in clashes that have pitted a collection of Kenya's tribes against Mr Kibaki's Kikuyu people, who are envied for their prominent role in Kenya's economic and political life. What started as political violence has now spiralled into a series of revenge and counter-counter attacks.

Major Mohamed Hussein Ali, Kenya's police commissioner, said: “We are urging people to remain calm and leaders should avoid speculating on the cause of the MP's death. Already the culprit has been arrested and will be arraigned in court tomorrow.”

The latest outbreak of violence forced the adjournment of the current round of talks in Nairobi aimed at settling the conflict. Ban Ki Moon, the United Nations Secretary-General, is due to arrive in Kenya today to help to bolster mediation efforts between Mr Kibaki and Mr Odinga. Adressing an African Union summit in Addis Ababa yesterday Mr Ban urged the two leaders to find a peaceful way out. “I call on the Kenyan people: stop the killings and end the violence now before it is too late,” he said. On Wednesday Jendayi Frazer, the top US diplomat for Africa at the summit, said violence had degenerated into ethnic cleansing in parts of the Rift Valley and that she wanted to see powersharing.

Alpha Konare, the African Union chairman, said yesterday that Kenya had once offered hope for the continent. “Today, if you look at Kenya you see violence on the streets. We are even talking about ethnic cleansing,” he said. “We are even talking about genocide. We cannot sit with our hands folded. If Kenya burns, there will be nothing for tomorrow.”

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