The arson squad is investigating a suspicious fire which damaged a mosque in Mirrabooka, which started just hours after Perth's Islamic community celebrated an important religious festival.

Fire crews were called to the scene about 12.20am.

The fire damaged chairs and a tree and smoke damage inside the mosque.

Police were still at the scene at 6.50am.

On Saturday night, the Islamic community in Perth's northern suburbs celebrated Eid, a festival marking the end of Ramadan.