Venice's "No Big Ship" campaigners, who oppose large cruise ships in the Venice lagoon, also took part in the protest, which started before dawn.
"The word is clear: the earth is on fire! It is the time to struggle, to act seriously, to reclaim climate and social justice," organizers wrote on Facebook.
The Venice Film Festival was due to end in the evening with the awarding of the Golden Lion for best film and other awards, including for best actor and actress.
Roman Polanski's "J'accuse," Noah Baumbach's "Marriage Story," Pablo Larrain's "Ema," Steven Soderbergh's "The Laundromat" and Todd Phillips' "Joker" are some of the films tipped for top prizes.
Legendary Rolling Stones frontman Mick Jagger was due to make an appearance in Venice as he stars in the festival's closing film, the out-of-competition "The Burnt Orange Heresy."
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