A group of gunmen kidnapped 317 girls from their boarding school in in the town of Jangebe, in Nigeria's Zamfara state last week.
"The abducted students and staff have regained their freedom and have been received by the Niger state government," Governor Abubakar Sani Bello says.
Local militia and residents say Boko Haram militants killed 10 people and injured 21 others with mortar shells in the volatile city.
Ruling party candidate Mohamed Bazoum is seen by many as the favourite after leading in the first round on December 27 with 39.3 percent of the vote. Polling stations opened at 0700 GMT and will close at 1800 GMT (8 am to 7pm) local time.
Nigeria’s president said he has dispatched security chiefs to coordinate a rescue operation for the students taken by the unidentified gunmen in Niger state.
Maiduguri city's three million residents plunged into darkness after a Daesh-linked militant group blew up transmission lines, in the third such attack in a month.
The fighting took place on Friday and Saturday in Salamat province, after cattle belonging to one herder ruined a field. The herders attacked as the farmers were taking the animals into an enclosure.
Four soldiers and seven hunters recruited to fight insurgents were killed when their vehicle hit landmines in northeastern Borno state.
Former interior minister Mohamed Bazoum, the ruling party's candidate, is the overwhelming favourite to succeed President Mahamadou Issoufou, who is stepping down after two five-year terms.
Boko Haram accused of storming a predominantly Christian village in Borno after security agencies warned the country of an increased risk of attacks during the Christmas holiday.
Boko Haram reportedly claimed the kidnappings but parents are less concerned about who was behind the abductions as they reconsider sending their children back to school.
The foiled abduction in Nigeria's northwestern Katsina state comes less than two days after the release of 344 schoolboys, who were kidnapped in the same area last week.
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