(Updates with new Yar'Adua quotes)

By Estelle Shirbon

ABUJA, April 26 (Reuters) - Nigeria's president-elect, Umaru Yar'Adua, has said he won disputed elections fair and square and received a first positive message from a foreign power, South Africa.

Foreign governments had been either silent or critical about the April 21 election that gave Yar'Adua a mandate to lead the African state of 140 million people for four years, until President Thabo Mbeki wrote to congratulate him.

"In his congratulatory letter ... Mbeki expressed South Africa's intention to forge closer working relations between the two countries," the Department of Foreign Affairs said in a statement late on Wednesday.

Backing from South Africa, the continent's biggest diplomatic power, will come as a relief to Yar'Adua after widespread condemnation of the elections.

Mbeki said he hoped parties and candidates would use only constitutional means to redress grievances.

Washington called the polls "flawed" and has not sent a message to Yar'Adua. EU observers said the election was "not credible" and "fell far short of basic international standards".

Observers from the regional West African bloc ECOWAS were among those who criticised the polls, but most African governments were expected not to openly contest the result.

The handover of power from President Olusegun Obasanjo to Yar'Adua on May 29 will be the first from one civilian president to another in the history of Africa's most populous country.

But many Nigerians say Obasanjo picked Yar'Adua, a little-known governor of remote Katsina State, as his successor because he wants to continue to dominate affairs of state.

"FAIR AND SQUARE"

Yar'Adua, who has denied he would be Obasanjo's puppet, defended his election victory.

"The authority and the legitimacy (of my government) arise from the constitution," Yar'Adua told Reuters in an interview late on Thursday.

"Once the election has been declared the result of that election is valid and legal unless it is invalidated by a court of law ... We expect all Nigerians to respect that and the international community to respect that," he said.

Opposition parties have rejected Yar'Adua's landslide victory and are calling for mass protests on May Day. Some have said they want the election cancelled and re-run under an interim government.

Yar'Adua has rejected the idea as unconstitutional. He has said aggrieved parties or candidates should seek redress through the courts and he would respect any court decision.

His main challenger in the election, former army strongman Muhammadu Buhari, said Yar'Adua could not escape blame. "He cannot dissociate himself from the rigging, whether as a candidate or as a governor of a state," Buhari said.

In the state Yar'Adua governs, Katsina, opposition supporters rioted during the elections over what they said was manipulation in favour of the ruling party.

Candidates have five weeks after the elections to take their complaints to electoral tribunals, but in many cases polling did not happen at all, so electoral fraud will be hard to prove. (Additional reporting by Paul Simao in Johannesburg, Daniel Flynn in Abuja and Tom Ashby in Lagos)

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