By Tom Ashby ABUJA, April 24 (Reuters) - The Nigerian opposition prepared on Tuesday for mass protests against flawed elections that gave the ruling party a landslide, but their chances of overturning the result looked slim. Rights groups slammed the vote that saw Umaru Yar'Adua declared president as a "charade", calling for the result to be cancelled and for new elections to be held. But there are only five weeks to go until President Olusegun Obasanjo must step down, marking the first civilian-to-civilian transfer of power in Nigerian history. Anything that undermines that historic transition would be an invitation to chaos -- or even a military coup -- an outcome even less popular among the opposition than a Yar'Adua victory, politicians said. "Anything to delay the handover of power would be unconstitutional," said Senate President Ken Nnamani, the third most senior state official, who has denied government accusations that he is behind a coup plot. "Once you have a result, as imperfect as it may be, the only body with jurisdiction to cancel an election is the electoral tribunal," he added. With a huge gap of 18 million votes between Yar'Adua and opposition leader Muhammadu Buhari, political insiders say a big legal upset is unlikely. Buhari has rejected the results and called for the National Assembly to impeach Obasanjo. Legislators say that is impossible in the time remaining. The other main opposition loser, Vice President Atiku Abubakar, has said the result "will not stand", but has not explained exactly what that meant. MASS ACTION "We are organising mass action," said Osita Okechukwu, who works for Buhari's campaign, adding that he was coordinating with labour unions and lobby groups and would spell out a timetable of rallies over the next few days. Oil prices climbed further above $68 as traders eyed the possibility of further disruptions to Nigerian oil output. Analysts raised concerns over the potential for more violence against the oil industry which has already curbed about 20 percent of total output from Africa's largest producer. Obasanjo responded to widespread criticism of electoral abuses in a nationwide address on Tuesday, appealing to any aggrieved losers to channel their protests through the courts. European Union observers said the vote was not credible, and the United States said it was flawed. Obasanjo instructed the electoral commission to make available evidence backing up its results and expressed hope that any differences would be resolved before May 29. Some of the rigging witnessed by observers, such as withholding ballots from opposition strongholds, is difficult to prove in court. Buhari has said he is reluctant to go back to the courts, having spent two years contesting Obasanjo's 2003 victory. Yar'Adua will be working to pacify his rivals. On Monday, he said he would reach out to the opposition and alluded to a possible "government of national unity". The deeply flawed election, which follows an equally flawed People's Democratic Party (PDP) primary that produced him, has made him many enemies both within and outside the ruling party. "If he can reconcile people and get them to believe he can do something in government, they might forget how he emerged," said one senior PDP member, asking not to be named. "But we could slide towards anarchy if he does not address the issues."
