Tense count follows Nigeria vote marred by glitches, sporadic violence

Nigeria continued counting ballots Monday in a closely fought general election in which technical glitches and sporadic violence frustrated the voting process and led to international concern of political interference but did not give way to the widespread violence of previous years.

The winner of the ballot in Africa's most populous and richest nation could be announced late Monday or on Tuesday, electoral officials said. Turnout among the 56.7 million registered voters appeared to have been high, with analysts saying the race between President Goodluck Jonathan and former military dictator Muhammadu Buhari is too close to call.

Despite violence linked to Boko Haram and sporadic unrest elsewhere, U.N. chief Ban Ki-moon and others praised the conduct of the vote on Sunday and called for calm to avoid a repeat of deadly rioting that followed 2011 elections.

But the U.S. and U.K. aired concern over evidence of political interference in the presidential vote. In a joint statement, Secretary of State John Kerry and British Foreign Secretary Philip Hammond noted "disturbing" signs but added that "so far" there was "no evidence of systematic manipulation."

Voters who were unable to cast their ballot because of technical problems on Saturday returned to the polls on Sunday, as the main parties traded accusations over the use of the technology.

Nigeria’s electoral commission said some 300 of the 150,000 polling stations would be open on Sunday to accredit voters after handheld devices to read biometric identity cards failed.

Attahiru Jega, the chairman of the Independent National Electoral Commission (INEC), said that the problems were limited to "only about 450" card readers.

Jega told a news conference in the capital Abuja on Sunday that he was concerned about allegations and the electoral authorities were investigating the complaints, which centered on opposition party agents being excluded from a vote-tallying meeting. 

Even before preliminary tallies were recorded, the opposition All Progressives Congress (APC) rejected the outcome in Rivers state, headquarters of Africa's biggest oil industry, and denounced the vote there as "a sham and a charade.”

The alleged irregularities sparked protests in affected areas.

On Sunday, hundreds of sympathizers chanted "APC" outside the electoral commission office in the oil city of Port Harcourt, prompting police to fire warnings shots. One group stoned a car they thought carried ballots.

Police in Port Harcourt on Monday fired tear gas at thousands of women supporters of the opposition coalition demanding the cancellation of the election in Rivers state.

The opposition coalition is demanding new elections in the oil-rich southern states of Rivers and Akwa Ibom, alleging irregularities including missing and false results sheets and electoral officials being replaced by government officials loyal to Jonathan.

The big fear is that violence may erupt once results are announced in the high-stakes contest to govern the nation.

More than 1,000 people died and some 65,000 were forced from their homes in northern riots after Buhari lost to Jonathan in 2011, according to the National Human Rights Commission.

Al Jazeera and wire services