PESHAWAR, Jan 30 (Reuters) - An explosion in a mosque killed at 19 worshippers and wounded dozens more in Pakistan's northwestern city of Peshawar on Monday, a hospital official said, and many of the casualties were police officers who had gathered for daily prayers.

The mosque was located close to a police housing block, and there were some 260 people inside when the blast occurred, according to police.

"It happened during prayers. A two-storey building has collapsed," an eyewitness told local news channel Geo TV, saying he was just outside the mosque when explosion happened.

"We received 19 dead and over 90 injured from the Peshawar Police Lines blast," said Mohammad Asim, a spokesman for the city's Lady Reading Hospital, referring to the neighborhood. "Many others are in critical condition.”

According to police there were more casualties buried beneath the rubble.

Footage from government broadcaster PTV showed police and residents scrambling to remove the debrise and carrying wounded people on their shoulders.

"A portion of the building had collapsed and several people are believed to be under it," police official Sikandar Khan said.