NAIROBI, Kenya — It took all of about 15 minutes for the slums to explode on Sunday after Kenya’s president was declared the winner of a deeply flawed election.

Thousands of young men came streaming out of Kibera, a shantytown of one million people, waving sticks, smashing shacks, burning tires and hurling stones. Soldiers poured into the streets to meet them. In other areas across the country, gangs went house to house, dragging people of certain tribes out of their homes and clubbing them to death.

“It’s war,” said Hudson Chate, a mechanic in Nairobi. “Tribal war.”

The dubious conclusion of one of the most fiercely-fought elections in Kenya’s history has pitched the country into chaos. Western observers said that Kenya’s election commission ignored clear evidence of vote rigging to keep the government in power.

Now, one of the most developed, stable nations in Africa, which has a powerhouse economy and some of the most spectacular game parks in the world, is the scene of tribal bloodletting. With the president, Mwai Kibaki , a Kikuyu, and the lead opposition figure Raila Odinga, a Luo, the election seems to have tapped into an atavistic vein of tribal tension that always lay beneath the surface in Kenya but up until now had not provoked widespread mayhem.

In Mathare, a slum in Nairobi, Luo gangs burned more than 100 Kikuyu homes. In Kibera, Kikuyu families loaded up their things in taxis and fled. Almost all the businesses in the country are shut. The only figures in downtown Nairobi, which is usually choked with traffic, are helmeted soldiers hunched behind plastic shields. Oily black clouds of smoke rose from the slums on Sunday evening and smudged out the sun.

As the riots spread, the government issued an order outlawing live media broadcasts.

“It’s a sad day for Kenya,” said Michael E. Ranneberger, the American ambassador to Kenya. “My biggest worry now is violence, which, let’s be honest, will be along tribal lines.”

Mr. Odinga’s supporters are unleashing their frustrations about the election, which was held on Thursday and initially praised as fair, against people they suspect supported the president, namely Kikuyus. The Odinga camp urged election officials to recount votes after exposing serious discrepancies between the votes initially announced on the day after the election versus the numbers that were then later entered into a national tally.

Mr. Odinga is Luo, an ethnic group that has long felt marginalized by the country’s Kikuyu elite that has dominated business and politics since independence in 1963. Mr. Kibaki is Kikuyu, and the voting so far has split along ethnic lines, with each candidate winning big in his home base.

Mr. Kibaki, 76, has been in government since independence in 1963 and is known as a courtly gentleman and economics whiz. But he is seen by many Kenyans as continuing an unfair political system that has favored the Kikuyu at the expense of Kenya’s 30-plus other ethnic groups. Mr. Odinga, 62, gained his popularity by tapping into those frustrations and building a multiethnic coalition.

It had been predicted that this election would be close, and the final results had Mr. Kibaki winning by a sliver, 46 to 44 percent. But that gap may have included thousands of invalid votes. The European Union said its observers in one constituency last week witnessed election officials announce that President Kibaki had won 50,145 votes, but on Sunday the election commission increased those same results to 75,261 votes.

“The election commission has not succeeded in establishing the credibility of the tallying process,” said Alexander Graf Lambsdorff, the chief European observer.

One Western ambassador said that Western diplomats tried for hours on Sunday to persuade the election commission to do a recount of the vote figures using original results but that the commission refused.

“This was rigged,” the ambassador said.

The election commission acknowledged that there were irregularities but said that it was not their job to address them.

The opposition, said the chairman, Samuel Kivuitu, “can go to the courts.”

The opposition has not indicated whether it would contest the results in Kenya’s courts, which are notoriously slow and corrupt. But it said it would have a swearing-in ceremony for Mr. Odinga on Monday and declare him the “people’s president.”