KISUMU, Kenya (AP) – Gunmen killed an opposition lawmaker in Nairobi early Tuesday, an attack that almost immediately stoked the ethnic fighting that has gripped Kenya since last month's disputed presidential election.

As with the gangs that have killed rivals and torched homes in western Kenya, groups of armed youths began gathering after the shooting in the capital's Mathare and Kibera slums. Since the Dec. 27 election, the death toll has soared over 800.

Two gunmen shot opposition lawmaker Mugabe Were as he drove to his house in suburban Nairobi, police said, adding they did not yet know if the political turmoil had motivated the slaying.

"We are treating it as a murder but we are not ruling out anything, including political motives," Kenyan police spokesman Eric Kiraithe said. "We are urging everyone to remain calm."

But a resident of Kibera, Teddy Njoroge, said houses were being set ablaze near a railway that generally divides members of President Mwai Kibaki's Kikuyu tribe from inhabitants of opposition leader Raila Odinga's Luo ethnic group. Flames and smoke rose from one area of Kibera.

"They have decided to revenge this MP," Njoroge said of the member of parliament.

Were was among a slew of opposition members who won seats in the December legislative vote, held at the same time as the presidential election.

The killing came as thousands of machete-wielding youths from both Kikuyu and Luo tribes hunted each other down in western Kenya's Rift Valley, burning homes, blocking roads with blazing tires and clashing with police who appeared overwhelmed.

In Kisumu town, where columns of smoke rose from burning homes on Monday, police took away one charred body.

"We didn't waste time, we had to kill him," said Stanley Ochieng, 25. He said they stoned the man, slashed him with machetes, then threw him to burn on their roadblock of burning tires because he was Kikuyu.

Hundreds of machete-wielding youths lobbed stones Tuesday at police, who responded by firing live bullets into the air.

On Monday in Kisumu, witnesses described seeing two people pulled from cars and stoned to death, while another was burned alive in a minibus.

"The road is covered in blood. It's chaos. Luos are hunting Kikuyus for revenge," said Baraka Karama, a journalist for independent Kenya Television in Kisumu.

The Rift Valley is home to the Kalenjin and Masai ethnic groups. British colonizers seized large tracts of land to cultivate fertile farms there. After independence in 1963, President Jomo Kenyatta flooded reclaimed farmlands with his Kikuyu people, creating deep-seated resentment that exists to this day.

Kikuyus also are resented for their domination of politics and the economy, a success cemented by endemic corruption and a patronage system where politicians favor their own ethnic group.

More than half the 255,000 people driven from their homes this month have been Kikuyus displaced in the fertile Rift Valley, an area famous for its farmland and wildlife frequented by tourists.

Kibaki and Odinga blame each other for the violence, trading accusations of "ethnic cleansing." Human rights groups and officials charge it has become organized.

"What is so alarming about the last few days is ... there's evidently hidden hands organizing it now," Britain's visiting minister for Africa, Mark Malloch-Brown, told reporters Monday.

He spoke after meetings with Odinga, Kibaki and their mediator, former U.N. Secretary-General Kofi Annan.

Annan announced in a statement released Tuesday that the "dialogue process" to help resolve the deadly dispute will start Monday.

Kibaki and Odinga had been asked to name three negotiators each to participate in the talks. They are under international pressure to form a power-sharing government.

In the past, Kibaki has said he is open to direct talks with Odinga but that his position as president is not negotiable. Odinga says Kibaki must step down and only new elections will bring peace.

In Naivasha, Kenya's flower-exporting capital on a freshwater lake inhabited by pink flamingoes, thousands of Kikuyus smashed shop windows with rocks and began looting. A handful of police fired into the air but were unable to control the mob of about 5,000.

Outside Naivasha Country Club, police were trying to rescue hundreds of Luos trapped by Kikuyus armed with machetes and clubs inset with nails.

"We're trapped," said Rose Achieng, who fled with her two children when looters ransacked her home Sunday. She and hundreds others had sought refuge next to the police station, beside the road outside the country club.

Police, apparently worried they could not protect them, started ferrying them in trucks to the town's walled prison compound, where more than 1,000 refugees already had gathered.

"If you stay we will kill you," Kikuyus yelled. When they surged forward, brandishing machetes, police fired into the air.

State Department spokesman Sean McCormack said Kenya has "gone from bad to worse, in terms of the violence."

European Union foreign ministers issued a statement indicating development aid could be pulled if Odinga and Kibaki don't agree to a power-sharing pact. But only about 6 percent of Kenya's budget comes form foreign aid, and the government has said it will not be blackmailed.