German police struck at a crowd of protesters with smoke bombs and water cannons as tensions boiled over in the streets of Hamburg, Germany, on Thursday, as leaders from the world’s largest economies gathered for the G-20 Summit.

Cable news outlets, estimating 10,000 protesters, streamed live video of police in riot gear hitting protesters with water cannons and dragging them from the smoke-filled streets.

President Trump’s meeting with German Chancellor Angela Merkel at the Hotel Atlantic Kempinski in Hamburg had concluded when the clashes began. The president had moved on to a dinner with Prime Minister Shinzo Abe of Japan and South Korean President Moon Jae-in at the U.S. Consulate General in Hamburg, his final meeting of the day.

CNN’s German correspondent Fred Pleitgen said the clash was set off when protesters wearing face masks approached a line of police, who demanded the face masks be removed.

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The protests throughout the day had been largely peaceful gatherings, but skirmishes broke out at about 2 p.m. EST, which is 8 p.m. in Germany.

Pleitgen described the protesters as “left wing” with an “anti-capitalist agenda.”

“Many of them highly critical of President Trump especially,” Pleitgen said. “There was quite a scene when President Trump's helicopter went over the demonstration ... the demonstration has been fairly peaceful, but now it seems as though things are getting a little bit out of hand.”

Police has stopped #Welcometohell protest march and people are squeezed in a tight passage. Danger of mass panic pic.twitter.com/8hU4VYy5nK — Gregor Peter (@L0gg0l) July 6, 2017

Dramatic scenes in #Hamburg. DW reporters are saying the water cannons are now being used at the #G20 protest. pic.twitter.com/atjLE1BmX2 — dwnews (@dwnews) July 6, 2017

Trump received a warm welcome for a speech in Warsaw, Poland, earlier in the day, where he extolled the virtues of Western democracy. But his relationship with German Chancellor Merkel, who is playing host to the G-20 summit, has been icy. The two leaders have starkly different views on trade, immigration and globalization.