Three windows were smashed at Norwich Ihsan Mosque early Saturday morning.
Police are appealing for witnesses to come forward following the criminal damage incident at about 6.30am at the building in Chapelfield East.
Around 11 panes of glass were damaged in the grade 2 listed building, which someone from the mosque said will cost 'hundreds' to replace to glass and leading.
Some worshippers were inside the building when the windows were smashed.
One of them, Rashid, said this was the first time anything like this has happened at the mosque.
'When I heard the glass being smashed I thought we were under attack,' Rashid said, adding: 'It is not nice to feel scared in a place of peace.'
He added: 'With what has been going on in the world lately I would not be surprised if it was a deliberate attack against us.'
It is unknown yet whether this was a hate crime or a random attack,
Uthman Morrison, a spokesman for the mosque, said the attack was a shock to the community.
He said: 'This is not something you expect to happen in Norwich and it goes against everything we know about living here.
'But we do not know if it was a hate crime or whether it could have been that people had too good a time and might not have known it was a mosque.'
Anybody with information should contact Sgt Dan Cocks at Bethel Street Police Station on 101 or call Crimestoppers anonymously on 0800 555 111.
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