By Estelle Shirbon

ABUJA, May 2 (Reuters) - Nigerian president-elect Umaru Yar'Adua promised on Wednesday to review the conduct of the disputed April elections that gave him his mandate with a view to delivering better ones in 2011.

Local and foreign observers said vote-rigging was so widespread that the elections were not credible, while the opposition has rejected the results. Yar'Adua has repeatedly said he believes he won fair and square.

"The next administration ... will examine as a matter of urgency the last elections ... so that we will raise the standard and quality of our general elections," Yar'Adua said at a ceremony during which he received a "certificate of election".

"I know that there is no country in the world where there are perfect (electoral) systems," he said.

Yar'Adua praised the Independent National Electoral Commission (INEC) for "a job well done" and said he would support it as it prepared for the next elections, in 2011.

But INEC's preparations for the 2007 elections and its conduct during the polls have been controversial. Observers and civil society activists said it was biased in favour of the ruling People's Democratic Party (PDP) and colluded with vote-riggers.

SACK

In the latest of a series of condemnations of INEC, Britain's All Party Parliamentary Group on Nigeria called on Wednesday for the chairman of the body to be sacked. It suggested Nigeria should approach the United Nations to seek an independent appointee to run INEC.

The PDP, which has ruled Africa's most populous nation and biggest oil producer for eight years, scored a landslide victory according to official results.

Yar'Adua was credited with 24.6 million votes against 6.6 million for his closest rival. The PDP won 28 out of 36 state governors' seats and an overwhelming majority in both houses of the National Assembly.

But European observers said the elections "fell far short of basic international standards" and were not credible, while the U.S. State Department said the process was "seriously flawed".

Monitors reported that in many parts of the country results were announced even though voting did not take place because ballot papers and result sheets went missing. They also saw ballot stuffing, theft of ballot boxes, multiple voting and intimidation in many places.

"I sincerely believe that I have the mandate of Nigerians," Yar'Adua said.

He has been governor of the remote northern state of Katsina for eight years. He was almost unknown in national politics until outgoing President Olusegun Obasanjo backed him as the PDP candidate in December primaries. Critics say Obasanjo means to use Yar'Adua as his puppet but both men deny this.