Australian officials say police fired pepper spray and beanbags at inmates overnight to quell a riot at the troubled Christmas Island immigrant detention center.

Police say up to 100 illegal immigrants began a violent protest late Thursday at the facility, pelting police and guards with broken concrete and other objects. One guard was slightly injured.

It was not clear what sparked the unrest although riots at the facility in March began as a protest against the lengthy processing of detainees' refugee status claims.

The situation was under control by Friday morning.

Detention is mandatory for boat people heading for Australia and most are taken to the Christmas Island center, which is overcrowded.