(Adds Britain, U.S., paragraphs 6-10)
By Andrew Cawthorne and C. Bryson Hull
NAIROBI, Dec 31 (Reuters) - Kenyan police battled protesters in blazing slums on Monday after disputed elections returned President Mwai Kibaki to power and triggered turmoil thought to have killed more than 100 people.
Riots hit the opposition’s western heartland near the border with Uganda, Nairobi’s shanty-towns and the resort of Mombasa on the Indian Ocean coast.
Reuters estimates about 100 deaths, based on witnesses, body counts and credible media reports. Local broadcaster KTN said the toll had reached at least 124.
A growing chorus, including members of Kenya’s electoral commission, expressed concern about the accuracy of the vote tallying. Each side accused the other of ballot-rigging after Kibaki was sworn in on Sunday.
Luos, who support defeated opposition leader Raila Odinga, targeted Kibaki’s ethnic Kikuyu group, Kenya’s largest and most economically dominant tribe, in much of the fighting.
In Kisumu, a pro-opposition western town, 21 bodies lay at a mortuary, witnesses said. Most had gunshot wounds. Some reporters were ordered to leave the premises by a man who said he was a government employee.
"We are in an undeclared state of emergency," civil society groups said in a statement.
The violence threatened to deter investment in east Africa’s biggest economy and damage Kenya’s reputation as an oasis of relative stability in a turbulent region.
British Prime Minister Gordon Brown spoke to Kibaki and opposition leader Raila Odinga on Monday.
"With both he expressed the UK’s concerns about the conduct of the elections but strongly urged both to work for unity and reconciliation," a spokesman for Brown said.
"All sides should exercise and work for a solution that reflects the will of the Kenyan people." Britain was the colonial power before Kenyan independence in 1963.
In Washington, State Department deputy spokesman Tom Casey said the United States had concerns about irregularities reporting the results, which he said should be resolved promptly through "constitutional and legal remedies".
On Sunday, the United States had congratulated Kibaki and called for Kenyans to "abide by the results".
"We urge all parties to restrain their supporters and reach out to each other to find a peaceful resolution in the interest of continuing to advance Kenya’s democracy and development. The United States is working with all parties to this end."
The African Union said it was "seriously preoccupied" by the events. Britain, France and other nations warned their citizens against visiting Kenya.
Kibaki urged reconciliation in a New Year message and he had a stern warning for troublemakers.
"My government will ... deal decisively with those who breach the peace by intensifying security across the country."
Kisumu has been the scene of the worst unrest. A senior security official said the mayhem eased only after police began shooting looters.
Water, food and fuel are in short supply in many parts of the country, and insecurity is paralysing transport and business. Some people took refuge in police stations.
Bewildered tourists, who contribute to an $800 million a year industry that is Kenya’s top earner, were stranded by delayed flights and unrest on roads to Mombasa airport.
On the Ugandan side of the border in Malaba, more than 300 Kenyans sheltered at a school because all the hotels were full.
In Nairobi, police fired teargas to turn back protesters chanting "No Raila, No Peace!".
Helicopters flew overhead as trucks packed with police and armoured cars mounted with water cannons raced to trouble spots.
Activists criticised a ban on live TV and radio broadcasts. The government said it was only trying to stop incitement.
Odinga again said he was Kenya’s elected leader, demanded Kibaki step down and called for a mass rally on Thursday in Nairobi’s main Uhuru Park, which means "freedom" in Swahili.
"For the last 48 hours the people have seen their nascent democracy shackled, strangled and finally killed," he said.
Kibaki’s Party of National Unity (PNU) accused Odinga’s Orange Democratic Movement (ODM) of "wanton rigging by ballot-stuffing" that gave their candidate 900,000 extra votes and turnout of more than 100 percent in some ODM strongholds.
Kibaki’s administration said it would declare no state of emergency or curfews, and would not deploy the military. "Police are handling the matter and the government expects the situation to normalise in the next few days," a statement said.
Within an hour of being declared winner, Kibaki, 76, took the oath in a snap inauguration on Sunday.
Chief EU monitor Alexander Graf Lambsdorff told Reuters the tallying process lacked credibility.
Four members of the Electoral Commission of Kenya (ECK) told a news conference that some information received from returning officers now "cast doubt on the veracity of the figures".
"If we find irregularities we’ll be the first to go to court and ask them to nullify what we received," said one, Jack Tumwa. "But under current law, ECK can’t nullify the election." (Additional reporting by Nicolo Gnecchi, Tim Cocks, Katie Nguyen, George Obulutsa, Helen Nyambura-Mwaura, Duncan Miriri, Wangui Kanina, Joseph Sudah, Guled Mohamed in Kisumu; Arjun Kohli in Mombasa and Francis Kwera in Malaba, Uganda)