The number of police officers injured in clashes with anti-G20 protesters has increased to 197, officials said.
Police said on Friday they were contending with another outbreak of violence in Hamburg's Sankt Pauli neighbourhood, the base of the Rote Flora (Red Flower) group that has played a major role in the protests against the summit, Efe news reported.
NDR television reported that stores had been looted and aired footage of successive police charges against bands of hooded youths.
Authorities continued to appeal to onlookers and others not involved in the protests to stay away from affected areas of the city of 1.7 million people.
The protests began on Thursday with the “Welcome to Hell” march, which included about a thousand hooded activists and was stopped by riot police with water cannons, as seen in images captured by epa photographers.
About 12,000 protesters had gathered in the early afternoon with plans to march to within 300 metres of Hamburg's Messehallen Convention Center, the venue for the gathering of leaders of the World's 20 major wealthy and developing nations.
The German government mobilised at least 19,000 police officers to provide security for the summit and officials have banned anyone, but residents and authorised personnel from entering a large section of the northern port city.
The summit went forward on Friday despite the disturbances, but the day's biggest story took place outside the formal session, as US President Donald Trump and Russian counterpart Vladimir Putin met in person for the first time.
As summit host, German Chancellor Angela Merkel condemned the violence.
“I have every understanding for peaceful demonstrations,” she said on Friday. “But violent demonstrations endanger human lives, they endanger people themselves, they put police officers and security forces in danger, put residents in danger, and so that is unacceptable.”