Hamburg - Several thousand demonstrators vowing to "Smash G20!" marched on Thursday with Hamburg police bracing for possible violence from a hardcore of several hundred hard-left activists.
Police put the number of participants at about 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 on 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.