Two mosques in Birmingham were targeted with heavy-duty catapults and ball bearings overnight, police said today.
Armed police officers have been deployed outside the mosques, they said.
West Midlands Police were called to Masjid Qamarul Islam in Small Heath area of the city at around 10pm (local time) and then to Al-Hijrah mosque nearby around 20 minutes later.
Police said they had recovered some ball bearings which appeared to have been fired from catapults, which smashed windows during evening prayers.
"Firearms officers were deployed as a precaution, but the bearings recovered are thought to be from a heavy-duty catapult," West Midlands Police said in a statement.
"The reason for the attacks aren't clear at this stage, but officers are continuing to patrol the area to reassure worshippers and local residents," the statement said.
Imam Usman Hussain, from Masjid Qamarul Islam, said people were "frightened" as windows were smashed.
"Worshippers thought they were under attack by someone with a gun. We heard a loud bang, people were very frightened, there was a sense of panic, but we don't know what the motive was," he said.
The incidents come in the wake of 29-year-old Sudanese-origin British Muslim, Salih Khater, being arrested on suspicion of terror offences in London after he smashed through security barriers outside Parliament in London on Tuesday, injuring three people.
A trustee of Birmingham Central Mosque, Nassar Mahmood, said Muslims are facing "unprecedented" levels of Islamophobia in the UK at present and urged the media not to speculate on the reasons behind the most recent suspected terror attack happened.
"We would ask you to be cautious of apportioning blame to the Muslim community," he said.
