Anti-globalisation activists have clashed with police across the German port city of Hamburg, leaving scores of officers injured.
Responding to a second day of protests as world leaders attended the G20 summit, police ordered in more than 900 additional officers from across the country.
Over 160 police officers were injured, dozens of activists had to be taken to hospital and more than 70 protesters were detained.
German Chancellor Angela Merkel condemned the violent protests as "unacceptable".
"I have every understanding for peaceful demonstrations," Mrs Merkel said. "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."
She thanked security forces for their work as the G20 met behind a heavy police presence in a no-go zone that was off-limits to most.
Thousands of officers in full riot gear patrolled as many as 30 different protest marches. Most of the demonstrations were peaceful and creative, but some rioters threw petrol bombs, iron rods and cobble stones.
More than 20,000 officers were on hand to guard the city’s streets, skies and waterways.
Police trucks blasted protesters back with water cannons, and officers physically dragged away a group holding a sit-in at the entrance to the summit grounds after they jeered and yelled at a convoy heading inside.
Anti-globalisation protesters also kept US first lady Melania Trump from joining the spouses of the other world leaders at the summit.
Violence seemed to be escalating on Friday evening as anti-globalisation activists forced their way into a closed train station by bending open the iron gates. Police responded by deploying a water cannon outside the Landungsbruecke station.
Protesters repeatedly tried pushing into the no-go zone - among them a group of 22 swimmers from Greenpeace who tried accessing the area from the Elbe River but did not succeed, police said.
Later on Friday, activists also attempted to get near Hamburg’s highly protected philharmonic hall, where international leaders were set to listen to a concert and have dinner together. Greenpeace boats blasted music toward the performance hall to disrupt the leaders’ meeting there.
Police condemned the "shocking criminal energy and high potential of violence" on display and tweeted a photograph of an officer with a bloody wound they said was caused by slingshot catapults.
Some officers had to be taken to the hospital, including one whose eye was injured when a firework went off in front of him.
The city’s fire department said 11 activists were severely injured and taken to hospital after falling off a 13ft wall after fleeing from a confrontation with riot police.
Police could not say how many activists were injured in the clashes, but the fire department said that as of Friday morning they had taken 60 protesters to hospitals across the city.