Thousands of the supporters of Nigeria’s main opposition party have held a protest against alleged voting irregularities in the country’s recent general elections, calling for the elections to be re-held.

The demonstration, organized by the opposition All Progressive Congress Party, took place on Sunday in the southern Rivers State’s capital, Port Harcourt.

The APC denounced the vote as “a sham and a charade,” adding that “whatever trash will (be) announced as the result of today’s election is not acceptable to us.”

Electoral commission chairman Attahiru Jega said the body was “concerned about what seems to be happening in Rivers State (where) there are many alleged cases of malpractices.” According to the chairman, among the irregularities being probed are allegations of electoral officials disappearing with voting result records and a number of administrators being replaced in the southern states of Rivers and Lagos.

Jega added that the commission was investigating the allegations and would respond to a party request for the general elections held in the state to be nullified and held again.

Protesters also voiced anger over the alleged killings of opposition campaign workers. The opposition coalition said a number of its members have been killed during the election and blamed “ethnic militias” working for President Goodluck Jonathan’s party, the People’s Democratic Party, for the alleged deaths.

People in the oil-rich West African country turned out en masse to vote in a presidential election on March 28 in a close race between incumbent Jonathan and former military chief Muhammadu Buhari. The elections were extended until Sunday on the account that ballot papers arrived late or imported card readers failed to recognize the fingerprints of the voters.

The country’s 56.7 million registered voters also cast their ballots in parliamentary elections, with 739 candidates competing for a place in Nigeria’s 109-seat Senate and 1,780 seeking election to the 360-seat National Assembly during parliamentary elections.

The vote was marred by violence, with more than 40 people killed in attacks by Boko Haram terrorists in the northeastern state of Borno and another 14 killed in attacks in neighboring Gombe State. Boko Haram, whose name means “Western education is forbidden,” and controls parts of northeastern Nigeria, had vowed to disrupt the polls by violent attacks.

The Takfiris have claimed responsibility for a number of deadly shooting attacks and bombings in various parts of Nigeria since the beginning of their activities in 2009, which have so far left over 13,000 people dead and 1.5 million displaced.