Protesters have clashed with police at a rally in central Sydney in support of the WikiLeaks website and its jailed founder Julian Assange.

About 70 officers, including mounted police and the riot squad, tried to keep the crowd of several hundred people on the footpath.

The protesters marched down busy George Street alongside a wall of police, chanting slogans and waving banners reading "Hands off WikiLeaks" and "We deserve the truth".

But when some of them tried to run on to the road, police stepped in and made a number of arrests.

The crowd reacted angrily and continued to march to Martin Place, where another arrest was made.

Police allege one man punched an officer and three protesters tried to block traffic at an intersection.

The man has been charged with assaulting police and the trio have been released with a penalty notice.

Greens MP David Shoebridge, who was among the protesters, says police could have handled the situation better.

"There were two ways the police could have dealt with it," he said.

"They could have worked with the organisers and there would have been a peaceful march to the consulate.

"The police refused to give permission to the organisers and instead there was a confrontation on the streets caused by that police intransigence."

Police say the protesters were refused permission to march through the city because the mandatory five days' notice was not given.

Inspector Chris Craner says police had a deal with the organisers that the protesters stick to the footpath.

"From the outset we've had a bit of non-compliance in relation to the issue of the scheduled one (march)," he said.

"We've been in negotiations with them, some of the organisers have been quite fine to talk to.

"There's always a crowd of people who try and disrupt certain events. Some people are here purely for a peaceful protest which is what we're happy with, we've facilitated that. Those that play up, end up being arrested."

But activist Pip Hinman says the protesters staged a peaceful rally and did not disrupt peak-hour traffic.

"The police I saw were grabbing people from the footpath. They were pulling people onto the street. At the same time they let out their dogs from their vans and that's where I saw a few people getting dragged away," she said.

She says they were trying to send a message to Prime Minister Julia Gillard.

"It was a message to Julia Gillard that Julian Assange is not a criminal," she said.

"If any charges have been laid, he has a right to have them heard in a court of law and not be tried by governments and the media."

Meanwhile, about 600 people protested in support of WikiLeaks in Melbourne, marching along Swanston Street to the British consulate.

Police say the protest was peaceful and no arrests were made.

- ABC/AAP