* Election commission results give Kabila lead
* Opposition calls for African mediation, rejects unity govt
* Kabila camp accuses opposition of preparing protests (Adds details, quotes)
By David Lewis and Jonny Hogg
KINSHASA, Dec 3 (Reuters) - Opposition parties in the Democratic Republic of Congo rejected partial results on Saturday that showed a lead for President Joseph Kabila in a Nov. 28 election, and called on African leaders to act to prevent violence.
The vast Central African nation held its second post-war election on Monday and the camps of both Kabila and veteran opposition leader Etienne Tshisekedi have said they are sure of victory, setting the stage for further trouble.
In a joint statement signed by major parties, including Tshisekedi's, the opposition cited irregularities in the way results were being released and said the electoral commission was "psychologically preparing the population for fraud".
"As a consequence, we reject these partial results and consider them null and void," said the statement, read by Vital Kamerhe, a former minister who is widely expected to come third in the poll and has committed himself to the opposition camp.
Partial results released by the electoral commission showed Kabila leading with 3,275,125 votes, while Tshisekedi trailed with 2,233,447 votes, based on 33.3 percent of polling stations counted.
The commission said it was forced to released the partial results after hackers managed to publish fake numbers on its official website that appeared to give Tshisekedi a strong lead.
Commission spokesman Mathieu Mptia rejected accusations of fraud and said the body was working transparently.
The tally included virtually no results from the capital Kinshasa, where Tshisekedi is confident of strong support. The percentage of votes counted varied widely by province.
Provisional results are due by Dec. 6.
Congo's government has beefed up security across the country in anticipation of the announcement of the results. Armed police patrolled the capital Kinshasa on Saturday.
Mobile phone text message services have been barred since Friday night. Security Minister Adolphe Lumanu told UN backed Radio Okapi on Saturday the government had blocked the services until further notice because of a spate of inflammatory texts.
Kikaya Bin Karubi, a senior member of Kabila's camp and Congo's ambassador to Britain, accused the opposition of readying the population to challenge the results in the street.
"We have someone who doesn't know democracy," he said.
Citing the examples of Kenya, Zimbabwe and Ivory Coast, where post-election wrangles led to violence, the opposition coalition called on the international community to act now, rather than wait until it was too late.
"We are calling on everyone to has an influence on the machine to resolve the problem now rather to wait and send in presidents ... while there is shooting in the street," Kamerhe said. "Let us do it now. We know the loser. We know the winner."
"We call on African elders or heavyweight presidents, because we want this to be a completely African mediation, to come and tell their counterpart here that stepping down is not the end of the world. We cannot burn Congo for one person."
NO UNTIY GOVT
New York-based Human Rights Watch said on Friday that 18 people had been killed in the run-up to the poll, with many shot dead by the presidential guard.
A U.N. Security Council statement also on Friday condemned violence in some parts of the country and "noted with concern the level of logistical and technical difficulties encountered ... during the voting process".
Jacquemain Shabani Lukoo, Secretary General of Tshisekedi's UDPS party, said there was "no question" of a government of national unity, and hinted at unrest if the authorities continue to perpetrate what the opposition believes is fraud.
"If they carry on like this there will be trouble, that's for sure, we will not let this lie," he told Reuters.
The election commission defied all odds to hold the election on Nov. 28, though it was marred by chaos, accusations of fraud and the lack of material that meant pockets of the country did not vote until days later.
The African Union and Southern Africa's SADC grouping have broadly given their approval to the vote. Other observer missions praised the Congolese for turning out in large numbers but said it was too early to say if the poll was free and fair.
Alex Ngwete, a respected Congolese blogger, reflected broad fears of trouble, especially in Kinshasa, if Kabila is announced winner of Congo's vote on Tuesday.
"Kinshasa will explode like a powder keg hit by an RPG, and that's not just a figure of speech," he wrote on his blog. (Reporting by David Lewis and Jonny Hogg; Writing by Bate Felix; Editing by Peter Graff)