Boko Haram fighters attacked poll stations in northeast Nigeria and a governor demanded elections be canceled in an oil-rich southern state Sunday as the count started for a presidential election too close to call.

Two electoral workers were killed Saturday in Boko Haram's campaign to disrupt the elections, chairman Attahiru Jega of the Independent National Electoral Commission told reporters.

Voting continued in certain areas on Sunday after technical glitches with new biometric card readers prevented some people from casting ballots on Saturday.

The high-stakes contest to govern Africa's richest and most populous nation has come down to a critically close contest between President Goodluck Jonathan, a 57-year-old Christian from the south, and former military dictator Muhammadu Buhari, 72, from the predominantly Muslim north.

Results are expected by late Monday. If there is no clear winner, a runoff must be held.

Suspected Boko Haram extremists attacked polling stations and destroyed election material in two northeastern towns Sunday, then advanced on Bauchi city, according to fleeing residents.

Soldiers engaged them in heavy gunfire, and a jet fighter patrolled skies above the city, they said.

Police spokesman Haruna Muhammad said security forces had halted the convoy of 10 vehicles holding "unidentified gunmen" at Dindima village, 10 kilometers (6 miles) from Bauchi.

But gunshots erupted in Bauchi before nightfall Sunday, and authorities declared a curfew.

Muhammad said the gunmen attacked polling stations in Kirfi and Alkaleri towns earlier in the day.

Boko Haram extremists killed at least 41 people, including a legislator, and scared hundreds of people from polling stations in three states in the northeast on Saturday.

Voters also are electing 360 legislators to the House of Assembly, where the opposition currently has a slight edge over Jonathan's party. Voting for 13 constituencies was postponed until April because of shortages of ballot papers, electoral officials said.

Nigeria's political landscape was transformed two years ago when the main opposition parties formed a coalition and for the first time united behind one candidate, Buhari. Dozens of legislators defected from Jonathan's party, including Rivers State Gov. Rotimi Amaechi.

Since then, Rivers has become a hotbed of political thuggery.

On Sunday, thousands took the streets of the state capital of Port Harcourt to protest alleged killings of opposition campaign workers and voting irregularities. Police in riot gear and armored cars moved in but the demonstration ended peacefully.

Police reported the electoral commission's office in Port Harcourt was bombed Sunday, and that three people, including a soldier, were shot dead Saturday. But the opposition coalition said scores of its members have been killed and blames "ethnic militias" working for Jonathan's party.

It also alleged that about 100 opposition members were detained by police on Saturday. Police did not immediately respond to those charges.

"We are concerned about what seems to be happening in Rivers State (where) there are many alleged cases of malpractices," electoral chairman Jega told a news conference. He said they were investigating and would respond to a party request for the elections there to be cancelled and rescheduled.

Jonathan's party charged that the card readers failed mostly in its strongholds and asked if this was contrived. "Sadly the damage that the failure of the card readers has caused to the fortunes of our supporters and party is immense," media director Femi Fani-Kayode said.

Jega said the commission had been "alarmed" by the number of card reader failures, but that the actual percentage was only 0.5, or 374 of more than 150,000 readers.

He told a news conference that other irregularities being investigated include electoral officials disappearing with results sheets and electoral officials being substituted in Rivers and Lagos states.

While voting has generally been relatively peaceful there are fears that violence will erupt when results are announced, as happened after 2011 elections when Jonathan defeated Buhari. More than 1,000 people died in violence between Muslims and Christians in the north.

The International Criminal Court has sent warnings to Nigeria that it is watching the elections and could prosecute anyone who incites violence.

Contested election results have led to mass ethnic bloodshed in Kenya and brought civil war to the capital of Ivory Coast.

Many Christian Nigerians attended Palm Sunday church services in which they prayed for a peaceful outcome for the elections.

Nearly 60 million Nigerians have cards to vote and for the first time there is a possibility that a challenger can defeat a sitting president in the high-stakes contest to govern Africa's richest and most populous nation.

A major campaign issue has been Boko Haram's Islamic insurgency. The failure of Jonathan's administration to curb the rebellion, which killed about 10,000 people last year, has angered many Nigerians.

International outrage has grown over another failure — the government's inability to rescue 219 schoolgirls kidnapped by Boko Haram nearly a year ago. The extremists have abducted hundreds more people since then, using them as sex slaves and fighters.

Nigeria's military declared on Friday it had cleared Boko Haram out of its strongholds in northeast Nigeria, a claim that could not be verified and seemed unlikely.

The Islamic uprising has exacerbated relations between Christians like Jonathan, who dominate the oil-rich south, and Muslims like Buhari, who are the majority in the agricultural and cattle-herding lands of the north. Nigeria's population of 170 million is almost evenly divided between Christians and Muslims.