The Supreme Court has unanimously ruled that Vice President Atiku Abubakar is free to contest this Saturday's presidential election, thereby making the election very uncertain. The court said the electoral commission had no power to disqualify a candidate from the election, also raising questions about the validity of the governorship elections where many candidates had been disqualified by INEC.
The strongest other opposition candidate has told the BBC he is not confident the election will be fair.
Opposition supporters took to the streets in parts of the country on Sunday, claiming the results had been rigged.
Tension is said to be high in many Nigerian cities, with a curfew imposed in Kano and sporadic shooting heard in Lagos.
Opposition presidential candidate Muhammadu Buhari said election officials and police had been biased against his All Nigeria People's Party (ANPP) in seven states.
"To get whole states where the elections were not fair, and where there was violence, is quite disturbing," he told the BBC.
"The PDP stalwarts are organising thugs, carrying away boxes, diverting other election materials."
The police say that 21 people died in election clashes over the weekend, although local media report up to 50 dead.
The BBC's Alex Last saw men storming a polling station and leaving with ballot papers in the southern city of Port Harcourt.
He says politicians often hire and arm young men to help them rig elections, attack rivals, steal ballot boxes and intimidate election officials or voters.
Political office is often seen as the quick path to real enrichment - a chance to tap into the country's huge oil revenues, especially in the oil-rich south of the country, our correspondent says.
Nigeria is Africa's biggest oil producer but unrest in the oil-producing Niger Delta has cut output by some 20%.
The PDP has won 26 of the 32 states declared so far.
The ANPP has won the northern states of Bauchi, Borno, Yobe and Zamfara.
Mr Abubakar's Action Congress has retained control of the state of Lagos, the commercial capital and the small Progressive People's Alliance has won in the south-eastern Abia State.
The Independent National Election Commission (Inec) said two other south-eastern states - Imo and Enugu - will have to re-run their elections because of irregularities.
Results are still awaited from Kano and Taraba states.
The announcement of the results in the most populous state, Kano, was suspended amid high tension with both the PDP and ANPP declaring victory.
Opposition supporters burned down government buildings in northern Katsina State, after the PDP candidate was declared the victor.
The state election was cancelled in south-eastern Imo State because of election malpractice.
The presidential contest should see the first time one civilian administration hands power to another since Nigeria's independence from Britain in 1960.
President Olusegun Obasanjo is standing down after two terms in office.
