Police say a female protester allegedly stomped on a constable's head during a violent protest in Auckland this afternoon.



A crowd of up to 300 anti-free trade activists descended on Sky City where the trans-Pacific trade talks are being held.



About 50 police faced off with the protesters who thumped against the Sky City doors, then set a series of boxes on fire. Fire officers doused them.



Protester Jax Taylor said the march was initially to present a petition protesting the secrecy of the Trans-Pacific Partnership negotiations to officials, but it quickly escalated out of control.



She said the group wanted to present the 750,000 signature petition to the head negotiator but when he refused to come down things got heated.



Police moved in and arrested several people. "It was nasty," Taylor said. "They were being very rough."The group then decided to burn the "symbolic" petition boxes brought with them in the middle of the street.



Fire were called to put out the blaze while police officers formed a line to push the protesters away from Sky City's front door.



"It was a bit dangerous," she said. "But if they'd just done what they said they were going to do by accepting the petition it wouldn't have got to that stage."



"All we want is transparency and accountability in the negotiations."



Police said that when they moved in, two officers were separated, attacked and kicked "numerous times".



Two arrests were made. One of these arrests was a female that allegedly stomped on a constable's head.



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



Witness Nick Shar said the protest was extremely frightening.



"The protesters were really aggressive they were trying to break down the doors, using the f-word an saying f*** the TPPA."



He said they also burnt an American flag.



"I've never seen anything like that. It was very very scary."



The Auckland meeting is the 15th round in the international trade talks involving 11 countries aiming to conclude the Trans-Pacific Partnership free trade agreement.



Inside Sky City are 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 a deal could extend corporate power into areas seen as national interests.



One issue causing alarm in New Zealand is how the TPP could affect the operations of public drug buying agency Pharmac.



Another key issue being debated and causing widespread concern is new intellectual property rules. The protest wound up at about 4pm. Police said there had been around 300 people present.



Organisers said the group was made up of several factions including Aotearoa Is Not For Sale, It's Our Future, the Occupy movement.

@mathewgrocott ANGER: The protesters gather outside Sky City.