Fourteen candidates, 810 voting delegates and thousands of other party members have poured into the capital.
The winner will lead the party into next year’s showdown against the ruling All Progressives Congress (APC).
Whoever emerges as the next president is expected to confront widespread insecurity and rising inflation.
Former Vice-President Atiku Abubakar, Rivers State Governor Nyesom Wike, former Senate Speaker Bukola Saraki and Sokoto State Governor Aminu Tambuwal are seen as the frontrunners in the PDP’s primary election.
The party was rocked this week by the withdrawal of its 2019 vice-presidential candidate, Peter Obi, who has also left the party, citing internal issues.
The PDP ruled Nigeria for 16 years after the return of democracy in 1999 until it was beaten by a coalition of opposition parties in 2015.
Since then, it has lost further ground to the ruling APC, especially in southern Nigeria where two of its state governors have defected to the ruling party.