Yasuyoshi Chiba, Getty Images
Residents of Kibera and supporters of presidential candidate Raila Odinga hold signs and chant "No Raila no peace in Kenya" after President Kibaki was declared the winner Sunday in the presidential race, the closest in the country's history.

NAIROBI, Kenya — Police fired tear gas and bullets Monday as they struggled to contain tens of thousands of opposition supporters accusing President Mwai Kibaki of stealing his re-election. The death toll in the demonstrations and ethnic clashes rose to at least 125 people, police and witnesses said.

Three police officers said they had orders to shoot to kill, while opposition supporters said they would risk death to protest what they called a stolen election.

The vote ignited smoldering resentment between Kenya's two largest tribes, with supporters of Raila Odinga, a Luo who officially came in second, clashing with members of Kibaki's Kikuyu. The head of Kenya's Red Cross said many of the dead were killed in ethnic violence across the country.

The Kikuyu comprise the largest ethnic group in Kenya, and are frequently accused by other tribes of monopolizing business and political power.

Thousands of people struggling to break out of Nairobi's burning slums surged back and forth under clouds of tear gas, and were pushed back with water cannons and baton charges. Police fired live rounds over their heads. Opposition supporters blocked a road into the city center with blazing refuse and tried to set a gas station alight.

Alex Busisa, 22, said police shot him and a friend after he walked out of his home near a demonstration. He spoke from a hospital bed after an operation for a gunshot wound to the stomach.

While politicians "could afford a plane to fly away ... it is the man on the ground who suffers, like me," Busisa said.

Odinga compared Kibaki to a military dictator who "seized power through the barrel of the gun," and he postponed a rally planned for Uhuru Park after police warned the opposition not to hold it. Odinga instead called on a million people to gather Thursday in the park — where protesters demanded multiparty democracy in the early 1990s.

"We will inform police of the march. We will march wearing black armbands because we are mourning," said Odinga, who had been ahead in early voting results and public opinion polls.

Kibaki vowed to step up security across the country to "deal decisively with those who breach the peace."

Inside Nairobi's Kibera slum, riot police fired shots into the air and tear gas into homes and businesses.

An Associated Press reporter saw a man who had been shot in the head being carried out in a blanket. Men around him said he had been shot by police. Police were not immediately available for comment.

Panicked residents called journalists to report ethnic gangs were roaming the narrow, sewage-filled alleyways of Kibera, seeking to avenge members of their tribe killed in overnight violence and setting homes on fire.

"Why are we burning these shops?" asked 26-year-old Abdi Ochieng as he watched his Luo neighbors cart away looted sheets of corrugated iron from smoldering Kikuyu businesses. "Kibaki does not own them. Neither does Odinga."

The violence has killed at least 125 people since Saturday across the country, police and witnesses said, although the tally was likely far higher. The head of the Kenyan Red Cross, Abbas Gullet, said that in many provinces Kikuyu homes had been attacked and families forced to seek refuge in police stations.

"They need food, water, blankets, but we cannot access them," he said. Enraged demonstrators had even demanded to know the ethnicity of Red Cross workers offering first aid to the wounded, he said.

Kibaki, 76, was sworn in almost immediately after the results were announced. Within minutes, the slums exploded into violence.

Suspicions over rigging were fueled by the fact that the opposition took most of the parliamentary seats in Thursday's vote, but Kibaki still won the election. A ban on live media broadcasts and partial suspension of the news have spurred the rumor mill, with gossip spread by text message and shouted from neighbor to neighbor across barbed wire fences and winding alleys.

Echoing previous statements by the European Union, the United States said on Monday it was concerned over "serious problems" during the counting of votes.

The State Department on Monday suggested the U.S. is not ready to recognize any winner in the questionable election.

"We do have serious concerns about irregularities in the vote count," Deputy Spokesman Tom Casey, said. "We call on the political parties in Kenya, as well as the Kenyan people, to avoid violence."

"I am not offering congratulations to anybody because we have serious concerns about the vote count," Casey said. "What's clear is that there are some real problems here and that those need to be revolved in accordance with their constitution and in accordance with their legal system."

Britain's Foreign Office issued a travel advisory for Kenya, warning people against nonessential visits to many parts of the country and urban centers because of the "serious and continuing outbreaks of unrest."

Kenya is one of the most developed countries in Africa, with a booming tourism industry and one of the continent's highest growth rates. Many observers saw the campaign as the greatest test of this young, multiparty democracy and expressed great disappointment as the process descended into chaos.

Kibaki's supporters say he has turned Kenya's economy into an east African powerhouse, with an average annual growth rate of 5 percent. He won by a landslide in 2002, ending 24 years in power by the notoriously corrupt Daniel arap Moi. But Kibaki's anti-graft campaign has largely been seen as a failure, and the elections have reopened festering resentment over tribalism and widespread poverty.

Associated Press writers Elizabeth A. Kennedy, Tom Maliti and Malkhadir M. Muhumed contributed to this report.