The Kenyan police have imposed a curfew after at least 10 people were killed in clashes in the central town of Nakuru.
The election has ignited long-standing ethnic tensions
Rival gangs of young men battled with machetes, metal bars, bows and arrows, while thick smoke billowed up from burning buildings.
"Nakuru town has been shut down... hundreds are injured in hospital," Kenya Red Cross head Abbas Gullet said.
Some 700 people have been killed in clashes between rival ethnic and political groups since disputed polls.
The violence comes despite hopes of progress after President Mwai Kibaki met opposition leader Raila Odinga for the first time on Thursday since December's disputed polls.
Armed groups have barricaded the main highway leading in and out of Nakuru and have been pelting stones at motorists who defy their orders to stop, says the BBC's David Ogot in the town.
Businesses have closed and Nakuru has been turned into a ghost town. Residents from the worst-affected suburbs of Shabab and Kaptembe have sought refuge in churches and the police station.
"The dead bodies and injuries are coming in. I cannot give you a figure now," the medical superintendent at Nakuru Hospital, Dr George Mugenya, told AP news agency.
Soldiers have been working with police to restore order and remove the barricades.
Some of the latest violence seems to be revenge by members of President Kibaki's Kikuyu community on groups seen as pro-opposition, such as the Kalenjins and Luos.
Kikuyus have been targeted elsewhere in the Rift Valley and many have fled to Nakuru.
"We have vowed that for every Kikuyu killed in Eldoret, we shall kill two Kalenjins who are living in Nakuru," said bus conductor Dennis Kariuki, according to Reuters news agency.
Our correspondent says suspected Kikuyu members of the outlawed Mungiki sect, dressed in either red T-shirts or hats have been patrolling the streets armed with machetes, clubs and other crude weapons.
"The police are unable to protect us so we have to take charge now," one young man told the BBC.
But AP reports that armed opposition supporters were also on the streets, shouting: "Raila is our president. Kikuyus go out of Rift Valley."
The curfew runs from 1900 local time (1600 GMT) until 0700 (0400 GMT).
Nakuru is capital of the Rift Valley, which has seen some of the worst violence in the past month.
Rival groups in the area have long-standing land disputes and these tensions have erupted, as ethnic, political and economic fault-lines have reinforced each other.
Former UN chief Kofi Annan has been holding further talks to try to end the crisis.
His spokesman said he would be meeting former President Daniel arap Moi, religious leaders and the head of the Electoral Commission of Kenya on Friday.
Mr Annan has no immediate plans to lead further direct talks between the election rivals.
People are fleeing Nakuru for Nairobi
Although Mr Kibaki and Mr Odinga shook hands on Thursday, the opposition were angered by the president's statement that he had had been "duly elected", saying he was not committed to meaningful talks.
But Kenya's Foreign Minister Moses Wetangula appealed to the Orange Democratic Movement (ODM) to give the discussions "a chance".
"We don't want to conduct negotiations from the rooftops," he told the BBC's Network Africa programme.
Mr Odinga's ODM say he was cheated of victory.
On Thursday, Human Rights Watch issued a report accusing opposition officials of helping to organise ethnic violence in the Rift Valley region.
ODM spokesman Salim Lone said HRW should provide concrete evidence before jeopardising mediation efforts. The party has denied previous accusations of ethnic cleansing.
"Raila Odinga has repeatedly condemned the killings," he told the BBC.
The unrest triggered by the election on 27 December has driven 250,000 people from their homes.