(CNN) -- German riot police clashed at times with protesters in Hamburg on Thursday evening as world leaders, including US President Donald Trump, arrived in the city ahead of Friday's G20 summit.
Police used water cannons as protesters threw bottles and smoke bombs during the anti-capitalist Welcome to Hell protest, taking place approximately 2 miles from the summit.
Firecrackers and bottles were also thrown at riot police who attempted to disperse the crowd as 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."
Julia Reusing, 27, from Frankfurt, was upset that police disrupted the demonstration.
"If this is all we can do just for showing our opinion and giving a statement -- and if the state forces are just shutting us up ... I mean what kind of state do we live in?" she said about authorities' initial moves to control the protests.
As night fell the situation calmed some but there was still tension. Police seemed to be targeting individuals they thought were troublemakers, not trying to disperse the large crowd.
Police tweeted they arrested at least one person for throwing a bottle. Three police officers went to hospitals with injuries and 12 others were hurt, police said.
The main body of protesters splintered and one group took over an intersection where a truck with a sound system blared music suitable for a house party.
Police stayed on the fringes, backed by two trucks with water cannons.
Mark Meyer is part of a grass-roots team of 100 or so lawyers who are volunteering to help protesters understand their legal rights if they are detained.
He told CNN that the police "wanted to crash and smash this demo from the beginning" and were looking for any excuse to do so.
He also said the red zone -- the no-go area near the summit -- is not legal and he will protest against it Friday.
The G20, founded in 1999, meets each year. It includes 19 countries and the European Union and 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 German Chancellor Angela Merkel's birthplace, leaders are expected to discuss climate change, terrorism and migration.
President Trump arrived in Germany earlier on Thursday after a visit to Warsaw, Poland.
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