Mr Obi, who came third in the election, filed his petition at the Presidential Election Petition Court in Abuja, at about midnight on Tuesday.
The Labour Party candidate sought five prayers in his petition.
Principal among his prayers is his call on the court to cancel the 25 February presidential election and order a fresh poll.
The petition, jointly filed by Mr Obi and the Labour Party as co-petitioners, joined the Independent National Electoral Commission (INEC), Mr Tinubu, Shettima Kashim (vice president-elect), and the All Progressives Congress (APC) as the respondents.
Tuesday, being the 21st day after the declaration of the results of the election on 1 March, is the last day he any other aggrieved candidates and political parties had to file their petition in court.
Tuesday, being the 21st day since the declaration of the results of the election on 1 March, is the last day he as well as any other aggrieved candidates and political parties have to file their petition in court.
Atiku polled a total of 6,984,520 votes in the election.
Grounds of the petition
Setting out the grounds for the petition, Mr Obi's lead lawyer, Livy Ozoukwu, said Mr Tinubu “at the time of the (presidential) election was not qualified to contest the election.”
Mr Ozoukwu, a Senior Advocate of Nigeria (SAN), argued that the president-elect “was not duly elected by majority of the lawful votes cast at the time of the election.”
The senior lawyer led Atiku's legal team when the former vice president unsuccessfully challenged the victory of President Muhammadu Buhari in 2019.
Among his five prayers, Mr Obi urged the Presidential Election Petition Court to “determine that at the time of the presidential election held on 25 February, 2023,” Messrs Tinubu and Shettima “were not qualified to contest the election. ”
He prayed the tribunal “to determine that all the votes recorded for” Mr Tinubu “in the election are wasted votes, owing to the non-qualification” of the president-elect and Mr Shettima.
“That it be determined that the 2nd Respondent (Mr Tinubu) having failed to score one-quarter of the votes cast at the Presidential Election in the Federal Capital Territory, Abuja, was not entitled to be declared and returned as the winner of the presidential election held on 25 February, 2023.”
Mr Obi prayed the tribunal to make “an order cancelling the election and compelling the 1st respondent (INEC) to conduct a fresh election at which the 2nd Respondent (Tinubu), 3rd respondent (Shettima) and 4th respondent (APC) shall not participate.”