An arson attack on a mosque is being investigated as a hate crime, police have said.
The fire at the NASFAT Islamic Centre on Droylsden Road in Newton Heath, Manchester, was discovered by police at 11.40pm on Sunday.
A Greater Manchester Police spokesman said investigations had revealed someone forced open a window and placed an accelerant inside the building.
Detective Chief Inspector Paul Walker said: "People are understandably shocked that someone has attacked a place of worship.
"We are investigating this as a hate crime which has damaged a place of worship used by members of the Nigerian community.
"Hate crime is often under-reported for a number of reasons, but people should have the confidence in coming forward, as no-one should be the subject of hate and intolerance.
"We are grateful to have very strong relationships and co-operation with different groups in Greater Manchester, and are working with partner agencies to support the community which has been affected."
A joint investigation with Greater Manchester Fire and Rescue Service is under way.
Five fire engines were called to the scene of the blaze.
Shamusideen Oladimeji, a spokesman for the mosque, said there had been two other arson attacks at the centre in the past three years.
He said there had also been other incidents in the past, including pigs' heads being thrown into the building, people urinating outside and verbal abuse.
He said: "We are not bad people. I don't know why this keeps happening."
Mr Oladimeji said the fire had caused "massive" damage to the classrooms inside the mosque.
He said: "We can't go in at the moment while the police and the investigators are there.
"We don't know how long it will take for the investigations to finish."
Police urged anyone with information about the fire to call 0161 856 9770 or the independent charity Crimestoppers anonymously on 0800 555 111.