Violent protest action left a police officer in hospital with neck injuries after his head was allegedly stomped on by a demonstrator.

Activists yesterday tried to force their way into SkyCity Hotel then lit a fire in the street during the march against ongoing secrecy in the Trans-Pacific Partnership free trade negotiations in Auckland.

Shocked bystanders said the aggression was extremely frightening.

"I've never seen anything like that. It was very, very scary," said witness Nick Shar, who was picking up his friend from the hotel.

Police say two of their officers were separated and kicked a number of times when they tried to stop the violence escalating. Up to 50 extra police and fire staff were brought in to deal with the situation.

There were around 200 protesters involved in the march.

A constable had his head stomped on and had to be taken to hospital for treatment, police said.

A woman was later arrested and charged with assault with intent to injure over the incident. A second woman was charged with disorderly behaviour.

Police said they were extremely disappointed at the reckless actions of some of the protesters.

"There were a number of protesters that were genuine in their approach and not violent. The violent protesters that were part of their group put their colleagues in unnecessary danger," a police statement said.

Protester Jax Taylor said the march was initially to present a 750,000-signature petition protesting at the lack of transparency in the Trans-Pacific Partnership negotiations to officials, but when the head negotiator refused to accept it, the gathering quickly escalated out of control.

"It was nasty," Taylor said. "[Police] were being very rough."

Veteran activist John Minto also blamed the police for the scuffle, saying tactics meant injuries were "inevitable".

"[Yesterday's] policing was way up there among the most stupid and provocative we've seen in 35 years of being part of various protests," he said.

"Police diplomacy, restraint and awareness were absent."

The Auckland meeting is the 15th round in the international trade talks involving 11 countries aiming to conclude the free trade agreement. Inside SkyCity were 500 negotiators from Australia, Brunei, Canada, Chile, Mexico, Peru, Singapore, the US, Vietnam and New Zealand.

The talks have attracted controversy because of their secrecy and concerns that a deal could extend corporate power into areas seen as national interests.

Those arrested will likely appear in court tomorrow.

- © Fairfax NZ News