Police are concerned "vulnerable groups of young people" could join Melbourne's climate protests on Wednesday evening and are warning "guerilla tactics" could turn the non-violent action into a threat to public safety.

North West Commander Tim Hansen also said climate protesters' refusal to let the authorities know what they were planning has forced police to suspend proactive campaigns around preventing robberies, carjackings and drug trafficking.

Protesters on pushbikes shut down parts of the city on Wednesday. Credit:Jason South

Mr Hansen said more police resources would be called upon late on Wednesday because of intelligence that disruption efforts by Extinction Rebellion protesters would ‘‘increase dramatically’’, which he warned could include protests at Melbourne airport and on public transport systems.

"What we've seen is passive resistance, so it's certainly not violent. Our intel from this afternoon onwards is that that could change," Mr Hansen said.