(Adds vice president stance in paragraph 9)

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 branded the vote that saw Umaru Yar'Adua declared president-elect a "charade", calling for the result to be cancelled and 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 threatens that deadline 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."

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, wants the National Assembly to set up an interim government for three months after May 29 which would hold a more credible poll, spokesman Garba Shehu said.

MASS ACTION

"We are organising mass action," said Osita Okechukwu, who works for Buhari, adding that he was coordinating with unions and would spell out a timetable of rallies in the next few days.

Oil prices climbed further above $68 a barrel on fears of instability in the world's eighth largest oil exporter. Militant attacks on Western oil facilities in the Niger Delta have already curbed output by 20 percent.

Obasanjo responded to criticism of electoral abuses in a nationwide address on Monday, appealing to any aggrieved losers to channel their protests through the tribunal.

The former military ruler told the electoral commission to make available evidence backing up its results and expressed hope that any differences would be resolved before May 29.

But 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.

But in Nigeria's ideology-free politics, loyalties often flow quickly towards incumbents.

AlertNet news is provided by