Boko Haram Islamist militants have attacked the north-eastNigerian city of Gombe, residents say.
Fighters overran a checkpoint on the edge of the city and explosions and gunfire could be heard, witnesses added.
Nigerian troops backed by a fighter jet were said to be trying to drive the militants back.
Nigeria has postponed elections due to be held on Saturday amid concerns over the insurgency in the north-east.
Gombe - capital of Gombe State - has previously suffered suicide attacks but correspondents say this is the first time Boko Haram has launched a direct assault on the city.
Reports said militants first attacked the town of Dadin Kowa, about 40km (25 miles) from Gombe.
Ground troops with air support then battled to keep the insurgents from entering the city.
One resident quoted by AFP news agency said Boko Haram gunmen were at a roundabout in the city centre, "firing indiscriminately and throwing pamphlets calling on people not to participate in the elections".
The report has not been verified.
The BBC's Abullahi Kaura in Abuja says residents have fled into the bush and into nearby hills.
Boko Haram controls a swathe of territory in north-eastern Nigeria.
Its insurgency has become a regional crisis with the four affected countries - Nigeria, Niger, Chad and Cameroon -agreeing to form a regional military force to try to contain the threat.
In response, the insurgents have stepped up their attacks in Nigeria and neighbouring states.