Approximately 60 people took part in the pro-Palestine protest at Lyttelton Port on Tuesday afternoon, and police said four people were arrested about 1pm after blocking traffic.
Protesters had blocked a tunnel and poured a liquid onto the road, a police spokesperson said.
Charges were being considered.
Palestine Solidarity Network secretary Neil Scott issued a statement saying members were "repulsed" by police actions at the protest, which he labelled "disgusting".
"The police arrested seven people and pepper sprayed many, including senior citizens protesting peacefully."
Scott said the group was 17 weeks into protests calling for a ceasefire in Palestine and for the government to take a stance against Israel's actions in Palestine.
Police "aggression" toward their activities had been increasing during that time, he said, and the group wanted an investigation into officers actions at the latest protest.
Police have been contacted for comment.
