At least 100 people have been killed across Kenya in violence blamed on the disputed presidential election.
A BBC reporter at a mortuary in the opposition stronghold of Kisumu saw about 40 bodies with gunshot wounds. A witness said police had opened fire.
There were running battles in Nairobi slums, and violence was reported in the coastal town of Mombasa.
Mwai Kibaki was officially re-elected president while Raila Odinga says he was robbed of victory by voting fraud.
Mr Odinga has called for a million-strong rally by supporters in Nairobi on Thursday.
Police banned his supporters from holding a mass alternative inauguration ceremony in the centre of the capital on Monday, a day after Mr Kibaki was sworn into office again.
In his New Year's message, the president urged reconciliation but warned that his government would "deal decisively with those who breach the peace".
On Sunday, Washington congratulated President Kibaki on his victory, but a state department official declined on Monday to repeat these congratulations when asked by reporters.
The US Ambassador to Nairobi, Michael Ranneberger, told the BBC there had been serious problems in a minority of constituencies but he said this did not necessarily mean the election had been rigged.
Shortly after first light, thousands of angry Odinga supporters started setting fire to buildings in Nairobi's vast Kibera slum while gangs of youths blockaded a nearby main road.
Violence has spread to the resort city of Mombasa
Police fired live rounds and used tear gas and water cannon as thousands of protesters tried to move out towards the city, the BBC's Josphat Makori said.
In the coastal town of Mombasa, angry crowds on the streets set fire to cars and buildings and at one point hundreds of frightened tourists were trapped at the airport, unable to leave by plane or road.
The situation there was going from "bad to worse" as hundreds of armed police poured into the streets to tackle marauding youths and angry demonstrators, the BBC's Odhiambo Joseph reported early on Monday evening, local time.
Our reporter said he had the impression that the security forces had actually been overwhelmed by the number of youths in the streets.
International news agencies have counted at least 100 deaths across Kenya - with some death tolls as high as 135 - either in clashes between protesters and security forces, or in ethnic violence.
- A hospital in the north-western city of Eldoret told AFP news agency it had recorded 24 violent deaths since Saturday, with most victims either injured by gunfire or machetes
- An AFP count puts the death toll in Kisumu at 53 and that in Nairobi's slums at 48
- Seven people were killed in Nakuru, in the Rift Valley, and three in the western city of Kakamega
- At least four people were killed in Mombasa
- At least two people were killed in the Nairobi slum of Korogocho
'Peaceful mass action'
Mr Odinga said he and his colleagues would not be intimidated by violence, and he urged people to join "peaceful mass action".
He told the BBC that people had taken to the streets in protest because their "democratic right had been usurped by a small clique".
Those killed in Kisumu include two women and three children, the BBC's Noel Mwakugu reports.
Police fired indiscriminately, even after the protesters started running away in the Kisumu suburbs of Manyatta and Nyamasira, an eye-witness told him.
Local police chief Grace Kahindi said she had no knowledge of any deaths.
A daytime curfew (0600-1800 local time, 0300-1500 GMT) was imposed in the town.
Some of the violence took an ethnic dimension with the Luo community seen as pro-Odinga and the Kikuyus viewed as Kibaki supporters.
European Union monitors were barred from counting centres in the Central Province, chief EU election observer Alexander Graf Lambsdorff told the BBC.
Results declared by the electoral commission in Nairobi from one constituency differed from those announced locally, he said.
He reported seeing altered voting forms where "all the changes favoured the same candidate".
Anomalies amounted to 20,000-25,000 votes in one constituency alone, he continued.
Mr Kibaki's national margin of victory was 230,000 votes.
Elections chief Samuel Kivuitu has admitted some problems, including a reported voter turnout of 115% in one constituency, the Associated Press reports.