Officers of the Nigeria police force and some supporters of the Peoples Democratic Party (PDP) clashed at the party headquarters in Abuja on Thursday.
The protesters besieged the area as former Vice-President Atiku Abubakar submitted his presidential nomination form.
The clash resulted in the officers firing gunshots into the air to disperse the protesters.
A witness said trouble started when some suspected thugs infiltrated the group of his supporters and started causing trouble.
They were said to have attempted to force entry into the premises. This resulted in the intervention of security operatives situation.
Abubakar is one of the front runners in the presidential race of the opposition party.
Among those who accompanied him to submit the form was Ben Murray-Bruce, a serving senator from Bayelsa state.
Abubakar has promised to support whoever emerges if he fails in his bid to secure the presidential ticket of the PDP.
Senate President Bukola Saraki; Aminu Tambuwal, governor of Sokoto state; former Senate President David Mark; Rabiu Kwankwaso, former governor of Kano state, are among those who have picked the presidential nomination form of the opposition party.
