Thousands of anti-capitalist protesters fought with police in Hamburg, Germany, Thursday ahead of the G-20 summit where President Donald Trump and other world leaders will be meeting. 

Police in Hamburg were seen using water cannons and tear gas to control the crowd of "Welcome to Hell" demonstrators that mounted to thousands by the evening. Violence broke out at a riverside plaza in the city, where people gathered earlier before starting their march through the city.

Black-hooded protesters attacked a police vehicle with bottles and bricks, breaking its window. Several people were also seen climbing over a wall as others stood in groups, holding up signs.

TRUMP, IN POLAND, VOWS, 'THE WEST WILL NEVER, EVER BE BROKEN'

Police said they repeatedly asked a group of hardcore demonstrators to remove their masks, but with no luck. Dozens of police officers created a blockade to separate the hooded protesters from the demonstration.

Police estimated there were 12,000 people in total at the demonstration Thursday. Authorities also said organizers of the demonstration, intended to be a march, called off the event once violence broke out.

More than 100,000 protesters are expected to be in the city during the two-day summit, according to police. Some 8,000 of them are considered to be part of Europe's violent left-wing group.

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

Trump arrived in Hamburg earlier in the day after delivering a speech in Warsaw, Poland, defending Western values and rallying against "dire threats" to civilizations.

"Just as Poland cannot be broken, I declare today for the world to hear that the West will never, ever be broken,” Trump said during his speech in Poland. “Our values will prevail, our people will thrive and our civilization will triumph.” 

THE G-20: WHAT TO KNOW ABOUT INTERNATIONAL SUMMIT

German Chancellor Angela Merkel and Trump met privately after his arrival, discussing issues that include North Korea, the situation in the Middle East, the conflict in eastern Ukraine and "some issues on the G-20 agenda." The discussion lasted more than an hour.

Secretary of State Rex Tillerson and German Foreign Minister Sigmar Gabriel also participated, according to a German government statement.

The Associated Press contributed to this report.