"There is no compelling moral, legal or political reason for us to join a government that we have told the whole world stole its mandate," Lai Mohammed, Action Congress spokesman, said on Friday.

'Stolen goods'

"If we partake in the government, we will be partaking in stolen goods, which is criminal. Besides, we will turn Nigeria into a one-party state as there will be no more opposition."

"If we partake in the government, we will be partaking in stolen goods, which is criminal"

Lai Mohammed, Action Congress spokesman
The main opposition All Nigeria People's Party (ANPP) and the smaller People's Progressive Alliance (PPA) have agreed to join with Yar'Adua.

The ANPP and the ruling People's Democratic Party said they had agreed to work together to review the electoral process, the constitution and a series of privatisations made in the last days of the previous government.

The election was the first transfer of power from one civilian leader to another in Africa's most populous nation but local and foreign observers reported widespread fraud and violence in many parts of the country before and during the polls.

EU observers said the elections "fell far short of basic international standards".

Yar'Adua's two main challengers, former army ruler Muhammadu Buhari of the ANPP and ex-Vice President Atiku Abubakar, the AC's candidate, are both contesting the result.

Official results gave Yar'Adua 24.6 million votes, Buhari 6.6 million and Abubakar 2.6 million.