Global leaders, among them US President Donald Trump, have gathered in the city for the meeting of the world's largest economies.
Police have begun using water cannons to disperse some of the thousands of protesters after apparently being hit with bottles and other projectiles. Tear gas has also been used, and local businesses have reportedly boarded up their doors and windows to prevent potential damage.
The Hamburg police have tweeted that at least one person has been arrested for throwing a bottle and that others have been stopped for wearing masks. According to the Hamburg police Twitter account, 76 police officers have been injured, and three taken to hospital for treatment.
Protesters in Hamburg have vowed to disrupt the summit. An estimated 100,000 people are taking part in the mass demonstrations.
There are colourful, and sometimes violent, protests every time the G20 meets. Yesterday, some 1,000 "zombies" took to the Hamburg streets in what was billed as a statement about political participation.
At the 2009 G20 summit in London, a bystander died after being roughly pushed to the ground by police forces. German football club St. Pauli allowed up to 200 people protesting against the G20 summit to set up camp on its Millerntor stadium in Hamburg, club spokesman Christoph Pieper said on Thursday.
