AUSTRALIAN police launched a brutal crackdown today on thousands of anti-war protesters outside a weapons convention in Melbourne.

About 1,800 police were deployed and used pepper spray, flash distraction devices and rubber bullets on protesters outside the huge biennial Land Forces International Land Defence Exposition.

At least 39 people were arrested for offences such as assault, arson and obstruction after exchanges during which the police accused the protesters of hurling bottles, rocks and horse manure.