KINSHASA - The democratic Republic of Congo's tense election standoff intensified yesterday after a team of international observers reported that incumbent Joseph Kabila's win was so flawed it lacked credibility.

Kabila, in power since 2001, was on Friday named the winner of the November 28 poll, but runner-up Etienne Tshisekedi immediately rejected the result and declared himself president.

Violent protests and looting erupted in the capital Kinshasa after the announcement. Police said four people had died in the unrest during the weekend.

Protests also spilled over to the Congolese diaspora in London and Brussels.

Election monitors from the Carter Centre, a non-profit organisation founded by former US president Jimmy Carter, added momentum on Saturday to Tshisekedi's refusal to accept the results, saying they "lack credibility".

"Multiple locations ... reported impossibly high rates of 99% to 100% voter turnout with all, or nearly all, votes going to incumbent President Joseph Kabila," the group said.

The Carter Centre said its observers gave a rating of "poor" to 40% of the 169 compilation centres where results were tabulated.

It reported irregularities including the loss of nearly 2000 polling station results in Kinshasa, a Tshisekedi stronghold, and chaos in the counting process ranging from ballots piled on the floor and stepped on to results sheets soaked in a rain storm then hung on sticks to dry.

The European Union and other international and local observers have also cited serious problems with the vote, ranging from disorganisation at polling stations to ballot box stuffing.

The election commission said Kabila had won 49% of the vote to 32% for Tshisekedi. Tshisekedi claimed his party's own count based on results taken directly from polling centres showed he had in fact won with 54%.

Government spokesman and Communications Minister Lambert Mende on Saturday threatened Tshisekedi with prosecution for the statement, which he called an "infraction of the law" and an "attack on the constitution".

Exacerbating the volatile atmosphere, national police chief Charles Bisengimana said security forces had fatally shot three looters and a woman had been killed by a stray bullet during the unrest.

UN broadcaster Radio Okapi said six had died in the unrest.

After Kabila's win was declared, protesters in Kinshasa set cars and tyres alight and threw stones at police, who responded with teargas and shots fired in the air.

A heavy security force presence, including police, presidential guards and 20000 soldiers on standby at military bases, put down the initial protests.

But sporadic unrest erupted again on Saturday despite heavy patrols by police and soldiers.

There were no reports of major violence in Lubumbashi, the restive capital of the southeastern mining province of Katanga, which had seen campaign clashes between rival partisans and a pair of deadly rebel attacks on voting day.

In London, police arrested 143 people on Saturday at an anti-Kabila demonstration that turned violent.

A protest in Brussels also turned violent on Friday, with police arresting two people suspected of throwing petrol bombs and detaining another 200.