A group of 80 young people paced up and down the pedestrian corso of Sydney's northern beachside suburb of Manly with their shirts off and some draped in Australian flags.

Local police and the riot squad rushed to the area about 2.30pm and arrested five 16-year-old boys before the mob dispersed nearly two hours later.

''We had a group of people from another location that came into Manly. They were very exuberant and loud and chanting the Aussie slogan [Aussie, Aussie Aussie ... oi, oi oi],'' NSW Police Superintendent Dave Darcy said.

The group walked up and down the 300m-long corso several times but things turned ugly when they jumped up and down on cars that were stopped at a traffic light.

A teenage girl suffered minor injuries when three people jumped onto the roof of one car, causing the back window to explode and shower her with glass.

Two males jumped on the roof and bonnet on another car and police were told others did the same to a taxi but no one was caught in the act.

''There were some instances where cars were damaged and we're investigating them,'' Superintendent Darcy said.

Police said a local shopkeeper caught two 16-year-old boys shoplifting and detained them. A struggle ensued but arriving police who arrested the pair released them with a caution after the man refused to give a statement to the officers.

Another youth was arrested and cautioned for offensive behaviour at Manly beach and two others were arrested for fighting. All three were aged 16.

Superintendent Darcy said the incident was exuberance gone wrong by a handful of people and the assaults were not racially motivated.

At Sutherland train station, in Sydney's south, a brawl erupted about 5pm, which resulted in one arrest and minor injuries to two people. Police also attended an unruly crowd about 5.30pm at Lake Ainsworth, near Byron Bay on NSW's far north coast.