Protesters injured at least 160 police officers, torched cars and lorries and broke into a locked-down metro station today, seeking to wrest control of the streets of Hamburg as leaders from the world's biggest economies met nearby.
The escalating violence prompted Hamburg's police to call in reinforcements from around Germany to help the 15,000 officers already deployed to the northern port city for the G20 summit.
A Reuters witness said protesters from the radical Black Bloc movement appeared to be trying to reach the city's riverside Elbphilarmonie concert hall, where leaders such as US President Donald Trump were due to attend a concert later.
The Black Bloc movement, which wants to overthrow capitalism, had some success in disrupting the leaders' discussions despite the ring of steel that surrounded them.
German Finance Minister Wolfgang Schaeuble cancelled an appearance in downtown Hamburg this morning due to security concerns. Police also declined to clear US First Lady Melania Trump's motorcade to leave her hotel to join in a tour of the city's historic harbour, her spokeswoman said.
"Thinking of those hurt in #Hamburg protests. Hope everyone stay safe!" the US president's wife tweeted afterwards.
Marine police units intercepted 22 divers from the environmental pressure group Greenpeace who had also been trying to reach the concert hall, police said.
German Chancellor Angela Merkel, host of the summit, condemned the violence.
The majority of an estimated 100,000 demonstrators in the city remained peaceful
"I have every understanding for peaceful demonstrations, but violent demonstrations put human lives in danger," she said.
Police said they used water canon to disperse protesters who broke into a riverside metro station that had earlier been sealed off. Nearby, two lorries were set ablaze.
Three officers required treatment in hospital, police said, noting that protesters had used slingshots as well as thrown bottles and stones. At least 70 people were detained and 15 were taken into custody, police said.
City police chief Ralf Martin Meyer told reporters that tight security around the conference area had caused the protesters to fan out around Hamburg, forcing police to request up to 1,000 reinforcements from across Germany.
"We are focusing on securing corridors to make sure that the path for (the leaders') convoys is clear," said Meyer.
In the violence, which first erupted during marches on Thursday, far-left protesters slashed the tyres of a car belonging to Canada's G20 delegation and smashed windows of Mongolia's consulate.
A police spokesman said only small numbers of far-left or anarchist protesters were involved in the disturbances, while the majority of an estimated 100,000 demonstrators in the city remained peaceful.
One of the many police helicopters patrolling the skies was nearly struck by a rocket flare, police said. On Thursday the pilots of another helicopter sustained eye injuries after a laser was directed against them.
Police said they continued to remove street blockades around Hamburg.