Climate activists are kicking off a fifth day of protests in Melbourne with a blockade at the global headquarters of mining giant BHP in the middle of the CBD.

The Extinction Rebellion protesters have described Friday morning's action as a "dress rehearsal" for a larger blockade later this month when the city hosts the International Mining and Resources Conference.

Extinction Rebellion activists are on their fifth day of protests. Credit:Jason South

More than 100 protesters have been arrested since demonstrations across Melbourne's CBD began on Monday.

Friday's action outside the BHP offices at Collins Street, near Russell Street, was set to begin about 7.30am.