The Australian Open erupted in ugly violence last night as police doused tennis fans with capsicum spray.

Panic gripped Margaret Court Arena as spectators gasped for air after coming into contact with the spray.

Many ran for the exits and the match had to be stopped.

Police used the spray to try to subdue fans of Greek player Konstantinos Economidis who became upset when police tried to evict members of their cheer squad.

Police believed they were leading racial chanting against Chile's Fernando Gonzalez.

Women and children were also hit by the spray. One woman said she had suffered an asthma attack and at least six people, including a mother and her two children, were treated by St John Ambulance for eye soreness and breathing problems.

Last year's Open was marred by crowd violence involving Serbian and Croatian fans, and tournament director Craig Tiley had warned there would be "zero tolerance" towards unruly fans.

But shocked fans accused the police of being heavy-handed, saying the use of the spray was totally unnecessary.

Supt John Cooke defended his officers, saying they'd feared for their safety and used the spray to keep aggressive patrons at bay.

They had "acted with restraint and professionally in a very difficult situation" and had defused a hostile situation without injury.

He said three men were ejected and interviewed for the offences of assaulting police, disobeying lawful direction and indecent language.

Supt Cooke said complaints had been made to police about offensive chanting, and warnings from officers had been ignored.

Gonzalez, who later won the match in three sets, admitted being scared when the police used the capsicum spray. "I was scared because the wind, it could come into my face," he said.

He said one or two things chanted by the Greek fans had been slightly offensive, but nothing serious. "It was fun, and it was actually a nice atmosphere."

Economidis said he was saddened by the security scare and disappointed his fans were involved. "It was sad that it happened. Nobody likes to see that.

"It was completely sudden."

Economidis said both he and Gonzalez were affected.

"It was unpleasant, because we had never seen something happen like this in the past. Not in my career," he said. "It was very disappointing for both of us."

"Witnesses said the incident happened midway through the second set, when police tried to eject two unruly Greek fans.

Herald Sun Online reporter Patrick Horan said things turned ugly when nearby fans objected.

"The crowd piped up a bit and started to yell out at the police, and all of a sudden a couple of officers have pulled out their capsicum spray and unloaded all over the crowd," he said.

"About three rows of people just scattered all over the place."

Elena Arapu, from Noble Park, said: "A few of my friends couldn't breathe. We all got it in our mouth and nose."

"We were all running out the doors to get away. There were little kids in the crowd and they copped it," she said.

Jess Reid, of Wonthaggi, who is asthmatic, said, "I have been down with St John of God for 45 minutes because I needed the mask to control the asthma".

Christian Colangelo, who was with Ms Reid, said a young family also was treated.

"There were two young kids, probably five and six years old. They were with their mother, and they were shaking," he said.

Greek fan Anna, 22, from Brunswick, said:"All we were doing was cheering on our player. It was completely unnecessary, and very scary."

Chilean supporter Manuel Munoz said: "All the Greeks did was chant the same way we did, and the police went totally overboard. If they thought they were doing the wrong thing, we were doing the wrong thing too."

A group of 15 Chileans later approached a group of about 20 Greek supporters, shaking their hands and hugging them, chanting: "We are friends."

The Metropolitan Ambulance Service said five fans were treated at the scene after being sprayed.