ABUJA, April 25 (Reuters) - The Nigerian opposition will join labour unions next Tuesday to protest against last week's flawed elections that gave the ruling party a landslide, it said after a meeting on Wednesday.

The opposition, led by former army strongman Muhammadu Buhari, has been searching for a plan of action since being declared losers of presidential and regional polls condemned by international observers as rigged.

The electoral body declared Umaru Yar'Adua of the ruling People's Democratic Party landslide winner after Saturday's presidential election in Africa's top oil producer.

President Olusegun Obasanjo has told any aggrieved losers to seek redress through the court before he hands over power on May 29, which should mark the first transition from one civilian leader to another in Africa's most populous nation.

"We urge Nigerians to utilise the occasion of May Day to identify with labour, reject sham elections on 14th and 21st April 2007 and call for their cancellation," the opposition coalition said in a statement, referring to the regional and presidential poll dates.

The programme of non-violent protests should start with prayer sessions on Friday, the statement added.

Opposition leaders are reluctant to make an outright call for mass protests because they fear they could be hijacked by violent or criminal elements, and lead to a heavy crackdown by the security forces.

The coalition also called for the disbandment of the electoral authority, and for fresh elections to be conducted by an interim government led by the Senate president that they want to see take over from Obasanjo on May 29.

But the interim government idea has already been rejected by the Senate leader, and is not supported by many in the opposition who see it as an invitation to chaos or even a military coup.

The statement was read by Ben Obi, running mate to Vice President Atiku Abubakar, who stood for the opposition Action Congress party but left for Britain earlier in the day for a medical checkup.

Buhari also attended the meeting, but did not stay until the end. Some in Buhari's All Nigeria People's Party are reluctant to swing behind the opposition strategy because they do not want to cancel regional elections that gave them control over five states, party sources said.

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