Reports from Nigeria say thousands of people poured onto the streets on Sunday to protest alleged killings of campaign workers.
They also lamented about voting irregularities in one of the countries big states, Rivers State.
Police had to come in to calm the situation managing to end the protests peacefully.
Police confirmed two people were killed during voting on Saturday, while the opposition put the figure at “scores” and alleged the vote there had been rigged.
By Sunday, thousands of opposition supporters took to the streets of the state capital, Port Harcourt, calling for the vote to be rescheduled.
The Situation Room, a civil society group monitoring the election, said it was “deeply concerned” about reports of “interference” in Rivers and neighboring Imo state.
Head of the electoral commission , Attahiru Jega said he was investigating irregularities as protesters called for a rerun in Rivers State following a tight vote over the weekend.
Jega Said reports of underage voting, electoral officials being substituted and the technical glitches that led to voting being extended to Sunday were all being looked into.
The electoral headquarters were burnt down in Rivers State an oil-producing state which has become a key battleground for the two main parties.
While voting has generally been relatively peaceful there are fears that violence will erupt when results are announced, as happened after elections in 2011 when Jonathan defeated Muhammadu Buhari.