A massive brawl between rival groups of teenagers and young men at a western Sydney shopping centre was sparked by a war of words on Facebook, police allege.

Police had been monitoring Facebook and other social networking sites days before two rival islander groups clashed at Westfield Mall in Carlisle Avenue, Mount Druitt, about 7.45pm yesterday in front of frightened shoppers.

About 100 young males, aged in their teens and early 20s, had armed themselves with baseball bats and homemade weapons but were met by about 65 police officers from the dog squad, the riot squad and other specialist officers.

Police say they restored order quickly but maintained a significant presence to ensure there were no further incidents.

Thirteen of the males, aged 16 to 22, were charged today with offences including affray, assaulting police and resisting arrest.

One person was taken to Nepean Hospital for treatment for minor injuries.

Acting assistant commissioner Wayne Cox of the North West Regional Command said the two groups were of Pacific Islander background. One came from the Mount Druitt area and the other from around Granville.

He said police had been tipped off by members of the public and monitored Facebook and other social networking sites.

"Those particular sites are not for violence or to bring out mob mentality," Mr Cox told reporters today.

"And they should be used for social networking, as they are designed for."

Facebook had provided police with some assistance, he added.

Police are reviewing CCTV footage from the centre and say they expect to make more arrests.

A 16-year-old boy was due to appear in Parramatta Children's Court today while three of the other males, aged 19 to 20, were due to face Mount Druitt Local Court today.

The others are expected to appear at the same courts on February 24 and 25.

Any witnesses who have not yet spoken to police are urged to contact Mount Druitt police station via Crime Stoppers on 1800 333 000.

Another victory for multiculturalism