The CPC's Muhammadu Buhari gained 32% of the vote, while Mr Jonathan got 59%.

The announcement of these results sparked widespread unrest in northern areas which had backed Mr Buhari.
An estimated 500 people were killed, thousands of people forced from their homes and some mosques and churches set on fire.

The election threatened to reignite Nigeria's religious and ethnic tensions, with most voters in the largely Muslim north backing Mr Buhari, while those in the mainly Christian and animist south supported Mr Jonathan.

"[The] election should be seen to be transparent, free and fair," CPC chairman Tony Momoh said after filing the law suit in the capital, Abuja.

"We have detailed election malpractices in the south-south, south-east, some states in the south-west geopolitical zones and even some states in the north," he said.

"We want the tribunal to nullify elections in these areas where there were flaws and conduct fresh elections in those areas," he said.

Most observers said the elections were the best organised since the return of democracy in 1999.

Previous elections have been marred by widespread and blatant fraud, with armed thugs employed by local political bigwigs storming polling stations and filling in all the ballot papers.

Mr Buhari, a former military ruler, has said he was cheated of victory in two previous elections.

Mr Jonathan was appointed to the presidency last year upon the death of incumbent Umaru Yar'Adua, a northerner whom he had served as vice-president.

Many in the north felt the next president should have been from their region, as Mr Yar'Adua died before he could finish his term.

Mr Buhari won most of the mainly Muslim northern states but nationwide only gained half as many votes as President Jonathan.

Analysts say the violence has more to do with poverty and economic marginalisation in the north than religion.