Brutal unrest across Kenya over President Mwai Kibaki's reelection left about 150 people dead on Monday -- some hacked to death – taking the overall toll to at least 185 killed in four days.

Police opened fire on some protesters and looters and many people were killed with machetes as ethnic tensions mounted.

Opposition leader Raila Odinga renewed his accusations that the presidential election was rigged and the United States withdrew its endorsement of the result.

Kibaki vowed to clamp down on the unrest.

"We have put enough police officers in the specific areas where the incidences of violence have occurred to ensure everyone is secure," he said in a message in which he appealed for "national healing" and reconciliation.

Odinga again rejected Kibaki's victory and urged his supporters to turn out for an alternative "inauguration" rally in Nairobi on Thursday. Police banned his plan for a rival swearing in on Monday and threatened Odinga with arrest if it went ahead.

The 76-year-old Kibaki overtook Odinga's early lead to win the election and his swearing-in on Sunday sparked a new round of violence.

Riots broke out almost immediately and police and mortuary officials said at least 75 people were killed in cities in western Kenya overnight and a further 48 in Nairobi's slum areas.

At least 24 people have died in election-related violence in the western town of Eldoret since Saturday, a hospital official said. Around 53 people were killed in Kisumu, an Odinga stronghold in the west, hospital officials said.

Ethnic rivalries have flared in the political tensions.

Six members of Kibaki's Kikuyu tribe were hacked to death Monday in the port of Mombasa, residents said.

"Whatever has happened to us, because Raila was not sworn in as president, we will avenge and start moving from house to house to kill the Kikuyus," one Mombasa resident said, before running amok with a gang of looters.

The Kikuyus, the country's largest tribe, responded to the deaths in Mobasa, killing three Luo, the second largest group, to which Odinga belongs. Another 10 people were killed in Mombasa in separate incidents, police said.

Foreign governments warned their nationals to avoid non-essential travel to the east African nation, while tour operators called off excursions for tourists already there.

In Eldoret, an official at the Moi Referral and Teaching Hospital said most of the people killed there had bullet and machete wounds.

The credibility of the election has been questioned by Britain, Canada, the United States and the European Union's election observers.

Washington initially congratulated Kibaki on his re-election but the US State Department on Monday withdrew the endorsement of the vote count made 24 hours earlier.

"We do have serious concerns, as I know others do, about irregularities in the vote count, and we think it's important that those concerns... be resolved through constitutional and legal means," State Department spokesman Tom Casey said.

"I'm not offering congratulations to anybody," he added.

The government has enforced a ban on live television broadcasts related to the election in what it says is an effort to contain the violence.

"We know there are skirmishes in many parts of the country. We are fully cracking down and fully responding to every situation," police spokesman Eric Kiraithe told AFP.

Kisumu police chief Grace Kaindi declined to comment on the death toll, but acknowledged that officers had opened fire on "looters" during the night.

The UN's top human rights official, Louise Barbour, called Monday on the Kenyan authorities to root out security force excesses.

UN chief Ban Ki-moon "urges the security forces to show utmost restraint" and "appeals to the population for calm, patience and respect for law," his press office said in a statement.

Amnesty International called for an independent probe into the killings of civilians.

"Those responsible for human rights abuses should be brought to justice without undue delay," the group said in a statement.

Police clamped a day-time curfew on the Kisumu, with an order to shoot violators.

According to police, hundreds of houses have already been torched in the western Rift Valley province and fresh riots and looting broke out Monday in Kibera, Nairobi's largest slum.

Odinga had planned to hold his alternative swearing-in ceremony on Monday, but was threatened with arrest if the rally went ahead. He predicted one million supporters would turn up for the new event on Thursday.

"We are calling for mass action, peaceful mass action," he told reporters.

The rage in the Odinga camp was in stark contrast to the celebrations that filled the streets of pro-Kibaki towns in central Kenya on Sunday, where revellers flooded local bars.

PHOTO CAPTION

A resident of the Kibera slum runs with a flag that reads "peace" past a burning building in Nairobi, Kenya.

AFP