Congo's election commission said it might not be able to announce a winner for last week's presidential election on Tuesday due to logistical delays, despite dispatching helicopters to ferry results from remote polling stations.

A delay in issuing full preliminary results from the vote in the vast central African state could further complicate an election already marred by violence, logistical problems and allegations of fraud.

In Kinshasa, tired election workers scrambled to collate results from the November 28 vote while diplomats sought to defuse tensions that have been building in anticipation of a result.

Partial preliminary results, representing nearly 70 percent of the ballots cast, give President Joseph Kabila a 10-point lead over his chief rival, Etienne Tshisekedi. But the opposition has said it will reject the outcome.

We want to keep to the date of December 6, but we've had some logistical problems, these have hampered us, electoral body spokesman Mathieu Mpita said. If we don't have the full initial tallies, we will release partial results.

The electoral commission set the deadline for a full preliminary count for December 6, the fifth anniversary of Kabila's inauguration and the day the opposition says marks the end of his constitutional term.

They know perfectly well that President Kabila cannot have even one hour more of his mandate after midnight, said Alexis Mutanda, president of Tshisekedi's campaign.

Mutanda said opposition party leaders were planning on how to respond if results are not announced.

U.N.-led diplomatic efforts are under way to allow a delay to the results if needed, according to sources.

Zambia's former president Rupiah Banda has said he is ready to help mediate any dispute arising from the vote but Kabila's camp has so far said he is not needed.

In a sign of tension, hundreds of Tshisekedi supporters gathered outside his residence on Tuesday shouting slogans of support and many carrying machetes, stones and petrol bombs, a Reuters witness said. Security forces kept a distance.

At least 18 people have been killed in election-related violence, according to U.S.-based Human Rights Watch, and Kabila's camp has warned the government would have to call in the army if protests become too chaotic.

We are not going to fall into the trap of massacring our own people, said Aubin Minaku, secretary-general of the ruling coalition backing Kabila. The police will work to maintain peace, but they will not fall into that trap.

The prosecutor of the International Criminal Court warned on Tuesday that those responsible for the violence in Congo would face justice, adding that the court was closely watching the situation of the ground.

Electoral violence is no longer a ticket to power, I assure you. It is a ticket to The Hague, the office of the prosecutor said in a statement.

Laurent Gbagbo, the former leader of Ivory Coast who refused to cede power after a presidential election last year, appeared in court at the Hague for the first time on Monday.

Congo's first locally organised and funded election since the official end in 2003 of years of war that killed 5.4 million people was meant to offer hope of greater stability.

But fears are mounting that a rejection of the results will unleash bloodshed. Tshisekedi enjoys broad support in Kinshasa, a city of 10 million people, as well as the two southern Kasai provinces, which have seen security beefed up in recent days.

REFUGEE CAMP

There was a heavy security presence on the streets of the capital on Tuesday, and some residents piled into boats to cross the Congo River into neighbouring Congo Republic, fearing renewed violence after the results.

The Congo Republic's government said it was preparing an encampment in case an outbreak of violence in Kinshasa triggers a larger refugee wave.

Banks across Kinshasa shut early on Tuesday and the city's normally chaotic central market was quiet, while the US embassy restricted staff to their homes until further notice.

I have my normal clients, but they aren't here. I don't know if they are scared, said Rose Nsele, sitting next to a pile of manioc leaves she had been unable to sell.

Congo's election commission defied the odds to hold the presidential and parliamentary vote on time on November 28. Often chaotic and at times violent, voting had to be stretched over three days due to delays in places.

International observers say the various steps of the counting process after the initial tally at polling stations had been poorly organised, with ballots and results sheets often lost or destroyed in the process.

Joseph Kabila, who succeeded his father Laurent Kabila after he was shot dead in 2001, won U.N.-sponsored elections in 2006 promising he would bring an end to a decade of chaos.

But his government has struggled against local and foreign rebel groups in the east and Congo remains among the most risky countries in which to do business.

(Additional reporting by Christian Tsoumou in Brazzaville, Chris Mfula in Lusaka, and Emmanuel Braun in Kinshasa; Writing by Richard Valdmanis; Editing by Bate Felix and David Lewis)