A suicide bomber has killed several people in northern Nigeria's Yobe state at a venue televising a World Cup match, residents and medics say.
A hospital worker told the media that truckloads of injured people are being treated in overcrowded wards.
"The injured people are so numerous I cannot count them," the worker said after the blast hit Damaturu town.
An emergency has been declared in three states, including Yobe, amid attacks by suspected Boko Haram militants.
Witnesses say the suicide bomber in a tricycle taxi detonated the explosives as people watched Brazil's match against Mexico on Tuesday evening.
"The military and police trucks that brought them in have made four return trips so far ferrying them in. Every single truck was full of the injured. And all of them are young men or children," the hospital worker said.
The worker said that the injuries suffered by people caught up in the blast were "horrific" and that troops were prioritising bringing in the injured for treatment, before returning to the scene of the blast to collect bodies.
Sanusi Ruf'ai, police commissioner for Yobe state, told AFP: "There was an explosion outside a soccer viewing centre here in Damaturu at around 8.15pm. "Our men have deployed to the scene but it's too early for us to give details."
Damaturu resident Mohammed Kurkure Yobe told the BBC that the venue where the attack took place is very popular and often crowded with people watching big events.
Damaturu is the capital of Yobe state, an area that has been devastated by attacks from rebel group Boko Haram which in April abducted more than 200 girls from a school in neighbouring Borno state.
Open-air viewing centres - where people pay to watch live football - are popular throughout Nigeria.
However, the Nigerian authorities have warned residents in some states to avoid public screenings of the World Cup, fearing militant attacks.
On Thursday, the north-eastern state of Adamawa ordered all venues planning to show live coverage of the football tournament to close, saying they had received intelligence of planned bomb attacks.
The states of Borno, Yobe and Adamawa have been under a state of emergency since May 2013. At least 2,000 people have died in the north-east since Boko Haram launched an insurgency in 2009.
Analysts say Islamist militant groups, including Boko Haram, have described football as un-Islamic.
On 1 June at least 14 people were killed in a bomb attack on a bar in Adamawa that was screening a televised football match. No group claimed responsibility for the blast.
In March, many people were also killed in explosions while watching football in a video hall in Borno's Maiduguri town. On both occasions, Boko Haram was blamed for the blasts.
Boko Haram - which means "Western education is forbidden" in the Hausa language - wants to overthrow the government and create an Islamic state.
The group captured international attention in April after it abducted more than 200 girls in Chibok in Borno state.
It is also suspected of killing at least 29 students in an attack on a boarding school in Yobe in February.