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In an unsavoury incident for the A-League, two people were injured as passion between fans of the two Sydney rivals boiled over at a pre-season fixture

A man was knocked out and a small boy hit with a rock after fans from Sydney FC clashed with rivals from new A-League franchise and city rivals Western Sydney Wanderers on Tuesday.

A brawl broke out just before half-time in the trial match between Sydney FC and New South Wales State League outfit Macarthur Rams after a flare was thrown by Wanderers supporters.

It is understood Sydney FC fans lobbed the flare back at them, sparking the altercation.

A Wanderers fan was knocked out in the ensuing scuffle and taken to nearby Campbelltown hospital in a neck brace.

A six-year-old boy was treated for bruising after being struck on the head by a rock as he entered the field for a kick at the interval.

The trial game was played in Sydney's south-west, unofficially considered Wanderers territory.

The brawl is unwanted publicity for the A-League as they prepare to launch the hastily cobbled together Wanderers into the 2012-13 competition.

Sydney FC boss Tony Pignata told reporters the incident was a "disgrace" and later tweeted: "Very disappointed in what happened tonight. Not needed and a few spoil it for everyone.

"Needs to be stopped now."

A-League chiefs are expected to take action against both clubs.