Police put the number of participants at some 8,000 at the beginning of the procession that was due to head towards and encircle the venue for the Group of 20 summit starting Friday.
A banner at the head read "Smash G20!" and "Welcome to Hell", the organisers' main slogan.
The hard-left "black bloc" included several hundred people dressed in black and carrying a huge black balloon.
Up to 100,000 demonstrators are expected before and during the two-day Group of 20 meeting bringing US President Donald Trump, Russia's Vladimir Putin, China's Xi Jinping and other leaders to the city.
Germany's second city, hosting its largest-ever international meeting, has deployed some 20,000 police, equipped with riot gear, armoured vehicles, helicopters and surveillance drones.
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