The blast Thursday, believed to have been caused by a bomb, occurred during evening prayers at the prayer center in Peshawar, capital of the northwest Khyber Pakhtunkhwa province, which has been the scene of much militant violence lately.
Provincial Health Minister Shaukat Yousafzai told CNN at least 67 people were injured in the explosion, but the extent of their injuries or their condition was not immediately known.
The Los Angeles Times said those killed included two children. The report said a state of emergency was announced at hospitals in Peshawar and officials appealed for blood donations.
The report said the mosque serves followers of the Tablighi Jamaat, an evangelical Sunni Muslim movement. Police estimated there were about 800 worshipers at the scene at the time of the blast.
Separately, CNN said authorities Thursday discovered and later defused a bomb at another mosque in Nowshera, about 30 miles east of Peshawar, police said.