A suspected petrol bomb exploded outside a Perth mosque while hundreds of worshippers were inside attending a prayer service, police said today.

No one was injured in the explosion, which occurred outside the Thornlie Mosque last night. Anti-Islamic graffiti was later found spray-painted on the wall.

"It is believed an accelerant was used to start the fire," Western Australia police said in a statement, adding that four vehicles were damaged, with one completely gutted by the fire.

An investigation was under way, police said.

"The Perth community was visited this evening by hate," Yahya Adel Ibrahim, a teacher at the mosque's Islamic college, wrote on Facebook.

"This, undoubtedly, is a criminal act of hate, but it is the act of a person or group not the greater whole," Yahya said, noting that worshippers remained at the mosque to finish their prayers.

Chris Tallentine, the area's local parliamentarian, visited the site and said, "I feel sick that you have to endure attacks like this. The truth is that this was an attack on all of us."

- dpa