Demonstrators tried to force their way into the Sky City conference centre in Auckland to protest against the Trans-Pacific Partnership trade talks.
Police called in reinforcements to prevent the anti-free trade area protesters from getting into the venue and arrested two people.
A series of fires were lit on the street outside before the protesters, numbering more than 150, police said, were moved on.
About 500 negotiators from 11 countries were at Sky City trying to conclude a free-trade agreement. The talks have been criticised for their secrecy.
Police said the fire service was called to deal with a fire that was lit in the middle of a road near Sky City.
A police statement said two officers were "separated, attacked and kicked numerous times".
One of the two people arrested "was a female that stomped on a constable's head", the statement said.