Police used water cannons and pepper spray to try to push back protesters who threw bottles, bricks and stones on Thursday ahead of the Group of 20 summit in Hamburg, Germany, according to The Associated Press.
Some demonstrators wearing black hoods broke a police vehicle's window.
Police told the AP that demonstrators were asked to remove their masks and that when they failed to comply, police separated them from the many other protesters.
They were taking part in the so-called "Welcome to Hell" protest, which was quickly canceled amid the violence. The AP reports the clashes broke out just as a march was getting underway from a riverside plaza used as a weekly fish market.
A building was covered with the slogan "Borderless solidarity instead of nationalism: Attack the G-20," says the AP.
Police had predicted there would be some 100,000 demonstrators. But German Interior Minister Thomas de Maiziere said earlier in the week that the city was bracing for up to 8,000 potentially violent protesters.
AP says Hamburg had boosted its security forces with 20,000 officers on patrol.
On Tuesday, police seized an arsenal of dozens of homemade weapons, according to The Telegraph. The newspaper reports,
" 'The militant far-Left are planning to organize the biggest black bloc ever,' Andy Grote, interior minister for the Hamburg government told German television.
" 'Black bloc' protesters wear masks to conceal their identity and protect themselves against pepper spray, as well as using improvised armor such as motorcycle helmets."
The protesters — many calling for peaceful demonstrations — were advocating for platforms ranging from fighting climate change to addressing worldwide economic disparities.
On Thursday, world leaders — including President Trump — were arriving in Hamburg ahead of the two-day summit's Friday kickoff.
CBS News reports that Trump was being sheltered away from the protests.
About an hour before the violence broke out, he met with German Chancellor Angela Merkel for closed-door discussions.
The German government said that the meeting lasted about an hour and that the two leaders discussed a range of issues including North Korea, the Middle East and the conflict in Ukraine.
Reuters reports that violent clashes are far from the image Merkel — who is running for a fourth term — wanted to convey. Merkel had hoped to show the world that major protests are part of a healthy democracy.
The annual G-20 summit brings together the world's biggest economic powers who "traditionally focus on issues relating to global economic growth, international trade and financial market regulation," according to the summit's website.
Leaders are also expected discuss the climate — a contentious issue since Trump pulled the United States out of the Paris climate accord last month.
This year's gathering has been highly anticipated, in no small part, because Trump is scheduled to sit down with Russian President Vladimir Putin on Friday.