An Australian teenager who was handcuffed and thrown to the ground by a policeman during Sydney's Mardi Gras had sworn at officers, the country's police say.

Assistant Police Commissioner, Mark Murdoch, said the initial charge against Jamie Jackson, 18, was "offensive language".

When police apprehended him for the charge at 10pm on Saturday night, a struggle ensued and witnesses started to film it on their phones.

In the video, which has amassed 150,000 views since Tuesday night, Jackson is thrown to the ground in handcuffs and an officer from the Fairfield local area command holds a foot on his back.

The footage has caused widespread outrage online and prompted the head of the Police Association, Scott Weber, to warn against making judgments or participating in ''trial by social media'' before all the evidence is compiled.

Onlookers in the video can be heard protesting and alleging the man had been handled violently before the video started rolling.

"We just saw you whack his head against the ground," a woman can be heard to say. "His blood is on the ground".

The footage does not show why Jackson first came to the attention of police but Murdoch told reporters that the initial charge was for offensive language.

Jackson was then charged with assaulting police and resisting .

New South Wales MP and gay rights campaigner, Alex Greenwich, said it was very concerning that a large police officer ''body slammed'' a teenage boy for swearing.

''If the only thing that the individual did was use offensive language then I think many would agree that the approach the police took to the situation and the body slam on the ground is certainly a heavy-handed approach to the matter,'' he said.

Weber said judgments on the video incident should not be made until an investigation is completed using extensive CCTV from the area and witness accounts.

He highlighted at incident at the Sydney Cricket Ground last year in which a police officer was filmed repeatedly punching a fan.

An investigation found that the officer's actions were justified and the fan had attempted to grab something from the police officer's holster.

Greenwich and the Mardi Gras board will meet with the Police Minister, Michael Gallacher, and the chair of this year's Mardi Gras, Peter Urmson, said: "We're not gonna let this get swept under the carpet".

An officer that told the witness to stop filming ''cos I said'' will be pulled aside and reminded of police protocol which encourages the public to film police, Murdoch said.

''This is a matter [we are] taking very seriously,'' he said. ''It's not about anything other than doing what we do, day in day out, and that is policing by consent and with the support of the community. We're not in the business of doing otherwise, we are not a third-world organisation.''

Murdoch said it was one of two incidents of police action being investigated internally after Mardi Gras.
In a second incident, the gay activist Bryn Hutchinson said up to five police held him down and kicked him, after he ignored their instructions to not cross a road.

"I was kicked several times," said Hutchinson, the former convenor of Community Action Against Homophobia, said. "I was handcuffed and had my face pushed into the ground.

"I had a police officer leaning on me. I told him I couldn't breathe. He said, 'If you can talk you can breathe".
Hutchinson has been charged with assaulting police.

- Sydney Morning Herald