Nigeria's main opposition party is asking a court to throw out some results from last month's presidential election, and for new polls to be held in some areas.
The Congress for Progressive Change, led by defeated candidate Muhammadu Buhari, said the voting process was flawed by irregularities in parts of both the north and the south.
The party's chairman, Tony Momoh, said Sunday there should be new elections held in those areas.
International and local observers have said the April 16 election was Nigeria's most credible vote in decades.
President Goodluck Jonathan, a Christian from southern Nigeria, defeated his northern Muslim rival, Mr. Buhari, who is a former military ruler.
Riots broke out after the vote, and a Nigerian rights group says at least 500 people were killed in Muslim-Christian fighting.
Nigeria's population of 140 million is split roughly evenly between Muslims and Christians. The country has suffered periodic bouts of Muslim-Christian violence, much of it taking place in the central city of Jos.
Some information for this report was provided by AFP and Reuters.