HAMBURG, Germany -- 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.

Thousands of people were taking part in a protest against the summit titled "G-20: Welcome to Hell."

Demonstrators gathered at a riverside plaza used for Hamburg's weekly fish market Thursday before setting off on a march through the city. The skirmishes came hours before the two-day gathering of the world's top economic powers gets underway Friday morning in Germany's second-biggest city.

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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.

German riot police run towards protesters during the demonstration during the G20 summit in Hamburg, Germany, July 6, 2017. Kai Pfaffenbach / Reuters

Organizers quickly called an end to the march after the violence broke out, police said. Skirmishes continued, with police advancing down the street with two water cannons while being pelted with bottles by a group of black-clad people.

A nearby building was plastered with the slogan, "Borderless solidarity instead of nationalism: attack the G-20." A small group on the roof set off fireworks. Police said windows at a furniture store and a bank were damaged. There was no immediate word on a number of arrests or injuries.

Many other groups are calling for peaceful protests and are pushing the G-20 leaders for action to fight climate change and address economic disparities in the world. Some are even calling for the dissolution of the G-20 itself so the United Nations becomes the platform for such discussions.

Hamburg has boosted its police with reinforcements from around the country for the G-20 summit, so that 20,000 officers are on hand to patrol the northern German city's streets, skies and waterways.

CBS Radio News correspondent Steven Portnoy reports that sirens could be heard across the city. President Trump, who arrived in Hamburg Thursday to attend the G-20 summit, was being sheltered in an area away from where the protesters were clashing with police.

More than 100,000 protesters are expected in the city for the summit, according to police.

Mr. Trump met with German Chancellor Angela Merkel in Hamburg Thursday. The pair shook hands and spoke briefly while looking directly at each other. They then left for closed-door discussions.

The German government says Merkel and Mr. Trump have discussed issues including North Korea, the situation in the Middle East and the conflict in eastern Ukraine. A brief government statement gave no further details of Thursday's discussion, which lasted a little over an hour, other than to say they discussed "some issues on the G-20 agenda." It said U.S. Secretary of State Rex Tillerson and German Foreign Minister Sigmar Gabriel also participated.

In the wake of Mr. Trump's recent decision to pull out of the Paris deal fighting climate change, the battle against global warming promises to feature prominently in discussions at the summit.

Merkel and Mr. Trump have met on three previous occasions. Merkel has visited the White House, and in May, the two participated in a NATO summit and the Group of Seven summit in Italy.