The violence that marred the Sydney A-League derby on Saturday will be investigated by police. 

Parramatta police were told on Monday that four Sydney FC fans were attacked by a large group of men, believed to be Western Sydney Wanderers supporters, in the Parramatta CBD after the match on Saturday night. 

Fairfax Media understands a small number of fans were set upon by a group of up to 30 men wearing black clothing bearing the insignia of a supporters' group while walking from Pirtek stadium to Parramatta station.

Two people are understood to have sustained considerable injuries and were taken to hospital. One man reportedly suffered a broken foot and another man a broken wrist. 

"It has been alleged four Sydney FC supporters were attacked by a number of Western Sydney Wanderers supporters after the game. An investigation is under way and any assistance anybody could provide will be appreciated," Inspector Peterson of Parramatta Police said.

Sydney FC boss Tony Pignata called for calm, saying A-League fans need to take a "chill-pill". Sky Blues defender Aaron Calver will not press charges after a Western Sydney fan slapped him on the face at the conclusion of Sydney's 4-3 win at Parramatta Stadium.

But the culprit is almost certain to cop a very lengthy ban from any further matches by Football Federation Australia, who have previously issued several fans with five-year bans for anti-social behaviour.

"Football Federation Australia is working with the Western Sydney Wanderers and all relevant authorities in relation to two security issues that occurred in and around Saturday night's match at Pirtek Stadium," an FFA spokesperson said on Monday.

"FFA can confirm the identity of a person who is alleged to have struck a Sydney FC player is known and that person will be subject to the National Code of Conduct and faces a ban from attending football matches under FFA's control.

"FFA is also liaising with police, security and both clubs to ascertain the full facts around an incident that has alleged to have taken place outside the stadium that has seen two Sydney FC fans injured. FFA would like to reiterate its zero tolerance to anti-social behaviour and is working with the authorities to identify the offenders and ensure they are banned from attending Hyundai A-League matches."

Several flares were also lit, with at least one being thrown towards the pitch, which is likely to result in multi-year bans for those involved. FFA is looking into all incidents.

There was no lack of action on the pitch in the seven-goal thriller which was decided by a late Terry Antonis winner. But a brilliant A-League spectacle has been tarnished by poor fan behaviour.

"All fans need to just take a chill pill to be honest," Pignata said on Monday. "We all want to support our teams win, draw or lose but after that let's get back to the world we live in. It's up to the FFA, they'll deal with it so I'll leave it to them to decide what needs to be done."

Pignata said Wanderers chief executive John Tsatsimas had called him to also express his disappointment. Asked if the only way fans would take notice was to punish Western Sydney, the Sydney FC CEO said: "I'll let you ask John Tsatsimas and the FFA that. I'm not going to answer that."

Calver had been dropped for the match in favour of Sebastian Ryall and was watching from a players' box high in the stands. The 19-year-old was making his way to the dressing room to celebrate with his teammates when he was assaulted by a Wanderers supporter, who was swiftly apprehended by security.

Pignata said while Calver was "shaken up" by the incident he didn't intend to file a formal complaint to police.