From the western opposition stronghold of Kisumu to the coastal city of Mombasa, in the capital Nairobi and the Rift Valley town of Eldoret, police clashed with gangs of youths, some of whom erected roadblocks and burnt tyres.

Police in Kisumu shot in the air and used teargas and batons to disperse a 1,000-strong crowd. Two men were shot dead, witnesses said. A Reuters cameraman saw a corpse in the street, with bullet wounds in the back and side.

In Nairobi, police chased a series of protesters through the central business district, firing teargas and live rounds in the air. The gas seeped into nearby offices.

Three youths were shot in the back of the leg as they tried to run from officers in the sprawling Nairobi slum of Kibera, one of Africa's biggest, a hospital administrator said.

"It was so crowded, a very narrow place. I was trying to escape and I got a bullet in my leg," one of the three, 18-year-old student Oscar Junior, said from his hospital bed.

Many Kenyans and expatriates in the capital stayed at home, shopkeepers boarded windows and traffic was light.

More than 600 people have died and 250,000 have been left homeless in the turmoil since Kibaki was sworn in after a December 27 vote that the leader of the opposition Orange Democratic Movement (ODM), Raila Odinga, says was rigged.

Police outlawed three days of rallies called by the ODM. Rights groups and Western governments, including the United States, have called on Kenya to lift the ban.

Gangs threw up roadblocks near Eldoret, in the Rift Valley area worst hit by the violence, while in Mombasa, police dispersed some 150 youths, scattering them briefly before they attempted to regroup, witnesses said.

"We want Kibaki to resign and pave the way for our rightful President Raila Odinga," said demonstrator Joel Oduor in Kisumu, coughing and crying from teargas.

GOVERNMENT CHALLENGES CRITICS

Kenya's political crisis has jeopardised its democratic credentials, angered donors, driven away tourists and hurt one of Africa's most promising economies.

Fuelling doubt over Kibaki's win -- officially by 230,000 of 10 million votes cast -- a senior U.S. official said on Wednesday it was impossible to know who won the presidency.

"We have done our own analysis. What it shows is that the result was extremely close and that whoever won probably won with no more than 100,000 votes at the most," Washington's ambassador to Kenya, Michael Ranneberger, told the Daily Nation.

"It is really not possible to say with certainty who won because the process was not transparent." But he called for power-sharing rather than a new vote or re-count.

Kibaki has solidified his position by naming the core of a new cabinet and convening parliament. But the opposition got a boost by winning the post of speaker in the assembly on Tuesday.

"Yesterday marked a turning point," Odinga told reporters. "The terrible wrong done Kenyans on 27th of December will not be able to stand. This is an illegitimate presidency."

ODM has the most legislators of any party, but does not have enough to win a no-confidence vote unless parties allied to Kibaki join it. Underscoring its narrow edge in parliament, ODM's choice for speaker won by just four votes.

Odinga and other opposition leaders headed in a convoy towards Nairobi's central Uhuru Park, ringed by riot police to prevent protests. Some of the leaders were tear gassed.

The United States and 12 other nations threatened to pull direct aid if the government's commitment to "good governance, democracy, the rule of law and human rights weakens".

But the government challenged critics to show proof.

"You have publicly claimed that the presidential results were flawed. Can you provide tangible evidence?" said a state advert in local media.

The opposition also stands accused of rigging and ballot-stuffing in its own heartlands during the polls.