Police used water cannon as protesters threw bottles and smoke bombs at the anti-capitalist Welcome to Hell protest, taking place approximately two miles from the summit.
Firecrackers and bottles were also thrown at riot police who attempted to disperse the crowd, while police helicopters circled overhead.
“I came here because in general I’m totally against the G20 summit,” Nicklas, a 21-year-old student protester, told CNN.
“…It’s the root cause or reason for what’s going wrong in the world. Wars can be bad but capitalism kills.”
The G20, founded in 1999, comprises the countries with the 20 largest economies in the world and meets each year.
The group — which includes 19 countries and the European Union — accounts for approximately 80% of global GDP; around two-thirds of the world’s population live in a G20 country.
During the two-day meeting, happening in Chancellor Angela Merkel’s birthplace, leaders are expected to discuss climate change, terrorism, and migration.
President Trump arrived in Germany earlier on Thursday following a visit to Warsaw, Poland.