HAMBURG, Germany — German police say organizers of a protest against the Group of 20 summit have declared the march over after trouble broke out at the start of the demonstration in the northern city of Hamburg.

Officers used water cannons and pepper spray to disperse black-hooded protesters at the event dubbed “G-20: Welcome to Hell.” Police say the masked protesters attacked them with bottles, stones and other objects and set fires in the street.

More than 100,000 protesters are expected in the city for the summit that takes place Friday and Saturday. Hamburg has boosted its forces so that 20,000 officers are on hand to patrol its streets, skies and waterways.

U.S. President Donald Trump and Russian President Vladimir Putin are among those attending the summit hosted by German Chancellor Angela Merkel.

German police have used water cannons and pepper spray to disperse protesters in Hamburg after being attacked with bottles and stones by some marchers protesting the Group of 20 summit.

Police say they repeatedly asked a group of hardcore anti-capitalist demonstrators to remove their masks Thursday evening, to no avail. They then decided to separate the group from the rest of the several thousand-strong demonstration.

Black-hooded protesters attacked a police vehicle with bottles and bricks, breaking its window.

The violence broke out near the start of the demonstration at a riverside plaza used for Hamburg’s weekly fish market.

More than 100,000 protesters are expected in Hamburg for the summit, which takes place Friday and Saturday.