NAIROBI, Kenya - President Mwai Kibaki was declared the winner yesterday of Kenya's presidential election and hastily sworn in, defying widespread concern over vote irregularities and sparking riots and tribal violence.

As smoke rose over parts of Nairobi, Kenya's emerging democracy also appeared to be smoldering. Before the chaotic election count, which saw returning officers disappear and European Union observers turned away without access to tallies, analysts and diplomats viewed Kenya as one of the most promising democracies in Africa.

But the politics of the Big Man still holds sway in many parts of Africa, with only a few cases of incumbent presidents losing power through the ballot box.

After Kibaki, 76, was sworn in for another five-year term, his challenger, opposition leader Raila Odinga, said that a ruling clique was trying to rob Kenya of its democracy, wiping away tears as he spoke.

Odinga, 62, said he would be sworn in as "people's president" in his own ceremony today and outlined plans for a parallel government. As he spoke, live television transmissions were abruptly cut.

There were reports of violence across the country. In Kibera, a Nairobi slum area and opposition stronghold, thousands of protesters armed with rocks, knives and machetes chanted, "No peace!"

Mobs burned shacks and kiosks and beat people up. Panic-stricken residents fled the area, shouting that gangs of youths were stoning cars, attacking people and robbing them. Police fired tear gas and live bullets to try to disperse the protesters.

But in Kibaki's strongholds, his supporters danced and sang.

The violence ran along tribal lines, as opposition supporters from the Luo tribe attacked those from Kibaki's Kikuyu tribe. Local media reported at least 13 people died, including several protesters shot by police. At least 70 had died in earlier election-related violence.

According to the official result, Kibaki won 4,584,721 votes and Odinga had 4,352,993. Odinga was well ahead in counting Friday, but Saturday saw the voting tally steadily tilt in Kibaki's favor, triggering riots in cities across Kenya.

"Kenyans will not accept the results of a rigged election," Odinga, the leader of the Orange Democratic Movement, declared earlier yesterday. "No force will stop Kenyans attaining what they want."

Odinga said his party's figures indicated that the vote had been rigged by 300,000 votes.

The parliamentary vote, also held Thursday, had shown a massive repudiation of the government. Twenty government ministers lost their seats.

The chief of the European Union election observers in the country, Alexander Graf Lambsdorff, reported evidence of irregularities.

"We regret that it has not been possible to address irregularities about which both the Election Observation Mission and the Electoral Commission of Kenya have evidence," he said in a statement.

He added that "some doubt remains as to the accuracy of the result of the presidential election as announced today."

A welder, Michael Mulama, 47, from Nairobi, said he was dismayed and puzzled by the result.

"Why has Mzee done this?" he said, using a Kenyan nickname for Kibaki, meaning wise old man. "Is the government happy when people are dying? This is really bad."

But at the swearing-in ceremony, just an hour after the result was announced, Kibaki insisted the election was free and fair and called for reconciliation and healing.

"I urge all of us to set aside the passions that were excited by the election process, and work together as one people with the single purpose of building a strong, united, prosperous and equitable country," he said.

With booming tourism and strong economic growth, Kenya is usually a haven of stability in one of the continent's most volatile regions. But tribalism determines voting patterns and often erupts into violence. In the lead-up to elections in 1992, 2,000 people died in tribal violence.

Throughout Kenya, opposition supporters reacted with disappointment and anger to Kibaki's victory.

"I am disappointed," said Joshua Owino, 32, a security guard in Nairobi. "I know Kenyans voted strongly for change and it has been denied.atorship."

Under Kibaki, Kenya has enjoyed stable economic growth, a booming tourist industry and free elementary school education, but his opponents say he did not deliver on his promise to deal with corruption, a big issue for voters. Odinga, son of Kenya's first vice president, is a wealthy businessman who promised to improve the lives of the poor.