ALSO READKabul Mosque blast: Death toll rises to 30 in deadly suicide attack Boko Haram attack leaves 13 dead, dozens hurt in Nigeria ISIS blows Mosul mosque where Baghdadi became 'caliph'; blames US Van rams into crowd near London mosque in 'potential terror' strike, 1 dead London attack: Finsbury Park Mosque has become totemic in terror politics
The blast happened during early morning prayers at the Madina mosque in the Unguwar Shuwa area of Mubi, some 200 kilometre (125 miles) by road from the Adamawa state capital, Yola.
"So far we have at least 50 dead from an attack at a mosque in Mubi," Adamawa state police spokesman Othman Abubakar told AFP.
"Several people were injured. We don't have the figure now because they have been taken to several hospitals for treatment.
"It was a (suicide) bomber who mingled with worshippers. He entered the mosque along with other worshippers for the morning prayers.
"It was when the prayers were on that he set off his explosives."
Asked who was responsible, Abubakar said: "We all know the trend. We don't suspect anyone specifically but we know those behind such kind of attacks."
The attack bore all the hallmarks of Boko Haram, the Islamist militants whose insurgency has left at least 20,000 people dead and more than 2.6 million others homeless since 2009.
Haruna Furo, head of the Adamawa state emergency management agency, and Musa Hamad Bello, chairman of the Mubi north local government area, both confirmed the attack.
They gave lower death tolls but both said the number of those killed was likely to rise.
Another emergency services official described the blast as "devastating". He said only that there were "high casualties".
Abubakar Sule, who lives near the mosque, said he had just returned home when he heard the blast.
"I was there when the rescue was on and 40 people died on the spot and several others were taken to hospital with severe and life-threatening injuries," he added.
"The roof was blown off. People near the mosque said the prayer was mid-way when the bomber, who was obviously in the congregation, detonated his explosives.
"This is obviously the work of Boko Haram."
Boko Haram briefly overran Mubi in late 2014 as its fighters rampaged across northeastern Nigeria, seizing towns and villages in its quest to establish a hardline Islamic state.
The town's name was changed temporarily to Madinatul Islam, or "City of Islam" in Arabic, during the Boko Haram occupation.
But it has been peaceful since the military and the civilian militia ousted them from the town, which is a commercial hub and home to the Adamawa State University.
In recent months, Boko Haram activity has been concentrated on the far north of Adamawa state, around Madagali, which is near the border with neighbouring Borno state.
Earlier this month, at least two civilians were killed when dozens of Boko Haram fighters tried to storm the town of Gulak but were repelled by soldiers.
There have been repeated suicide bombings in the area, which lies not far from the Sambisa Forest area of Borno, where the militants had a base.
Boko Haram fighters are also said to be hiding in the Mandara mountains, to the east of Adamawa state, which forms the border with neighbouring Cameroon.