Presidential and parliamentary polls are scheduled for February 14, 2015 but tensions are already rising between President Goodluck Jonathan's ruling party and the main opposition.
Department of State Services (DSS) spokeswoman Marylin Ogar told AFP that Mujahid Asari-Dokubo, leader of the outlawed Niger Delta Volunteer Force, had been invited for questioning.
"He is still with us," she added.
"His invitation is based on our earlier warning to all Nigerians to desist from making inflammatory statements that could threaten (the) peace and security of the nation."
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The activities of Asari-Dokubo, a Muslim convert formerly known as Melford Dokubo Goodhead Jr, led to substantial drop in Nigeria's oil production, prompting the government to jail him.
He was released in 2007 after renouncing violence but has lately been drumming up support for Jonathan, who also hails from the Niger Delta region.
"The last time Obasanjo arrested me, my arrest reduced Nigeria's oil production to 700,000 barrels per day," he was quoted as saying in the local media.
Last week, the DSS quizzed Nasir El-Rufai, a former minister and key opposition figure, over comments he made that were interpreted as a possible incitement to violence.