Fleeing residents said scores of the extremists are advancing on Nigeria's northeastern city of Bauchi and soldiers are engaging them with heavy gunfire.
Fighter jets flew above the city and police spokesman Haruna Muhammad said security forces had stopped a convoy of 10 vehicles of "unidentified gunmen" at Dindima village, 10 kilometers (6 miles) from Bauchi.
Residents said the convoy is carrying Boko Haram fighters who first struck at about 7:30 am (local time) at Alkaleri, a town about 120 kilometers (75 miles) east of Bauchi.
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 yesterday.
In electoral violence elsewhere, three people including a soldier were shot and killed in southern Rivers state and police said two car bombs exploded at polling stations in the southeast but no one was injured.
Elections were extended into today in about 300 of the country's 150,000 polling stations, including some areas of Lagos, Nigeria's megacity of 20 million on the Atlantic coast, according to the country's electoral commission.
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.
The front-runners among 14 candidates are President Goodluck Jonathan, a 57-year-old Christian from the south, and former military dictator Muhammadu Buhari, 72, from the predominantly Muslim north.
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.
Dozens of legislators defected from Jonathan's party.
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.
