Nigeria's opposition leader Atiku Abubakar who came second in a February 25 presidential poll took to the streets on Monday along with his supporters to contest the election results.
The electoral body last week declared Bola Tinubu of the ruling All Progressives Congress (APC) as the winner to take over from President Muhammadu Buhari in May.
Almost 25 million people cast ballots in a vote that was largely peaceful but marred by long delays and technical glitches, angering voters and opposition parties who have claimed massive vote-rigging.
Abubakar's supporters and members of his Peoples Democratic Party (PDP) dressed in black to walk to the Independent National Electoral Commission (INEC)'s headquarters in Abuja and submit a petition, claiming electoral fraud.
INEC has "direct involvement in aiding and abetting the monumental rigging and manipulation of the election results in favour of the ruling party," said Iyorchia Ayu, the PDP chairman.
"It is really an appalling situation," said another PDP leader, Baraka Sani, surrounded by protesters with banners saying "Save our democracy" and "INEC is corrupt".
The commission has acknowledged technical difficulties on the day of the vote but rejects claims of fraud.
The Labour Party's Peter Obi, who came in third in the election, has also rejected the results, and said he was going to court to prove to Nigerians he won the presidential race.
Running for a sixth time, "Atiku", as Abubakar is known, has not clearly said if he is going to court, but he is consulting lawyers to "decide the next line of action".
Nigerian elections have often been marred by fraud allegations.
In a bid to improve transparency, INEC this year introduced biometric voter identification for the first time at the national level as well as IReV, a central online database for uploading results.
But some voters and opposition parties said failures in the system when uploading tallies allowed for ballot manipulation and disparities between the manual and online results.
Problems with the new technology also caused huge delays and queues, discouraging some from voting.
With registered voters numbering 93.4 million, turnout was just over 27 percent -- even less than in the 2019 election.
INEC is set to address some of the concerns raised by parties and voters before key governorship and local assembly elections on Saturday.