Thousands of anti-capitalist protesters clashed with police Thursday in Hamburg, Germany, ahead of the G-20 summit.

US President Donald Trump and other world leaders are expected to meet at the summit Friday.

The demonstrators, who gathered two miles away from the site of the summit, called their gathering the 'Welcome to Hell' protest.

Protesters threw firecrackers, bottles, and smoke bombs at riot police, who attempted to disperse the crowd with water cannons and tear gas.

The window of one police vehicle was broken when protesters threw bricks at it.

President Trump arrived in Hamburg after visiting Poland, where he spoke about threats to western civilization.

"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 summit is a gathering of the leaders of the 20 largest economies in the world. Leaders are expected to discuss climate change, terrorism, and migration.