Fires burned across downtown Hamburg on Friday evening during the second day of clashes between protesters and police while leaders of the world's 20 most powerful countries gathered nearby.

Key points: Almost 200 police were injured in the clashes which saw stores looted and cars set on fire

Almost 200 police were injured in the clashes which saw stores looted and cars set on fire Authorities say that of 100,000 demonstrators, only a handful were involved in violence

Authorities say that of 100,000 demonstrators, only a handful were involved in violence Angela Merkel says the violence has put lives in danger

Police said at least 196 officers were injured in the clashes, with 83 protesters temporarily detained at the scene and 19 taken into custody.

Protesters torched cars, looted stores, lit firecrackers and blasted Jimi Hendrix songs in a vain bid to drown out the classical music playing at the Elbphilarmonie concert hall across the Elbe River, where world leaders were at a performance of Beethoven's Ninth Symphony.

Reinforcements poured in from across Germany but police were unable to prevent thousands of protesters from fanning out across the northern port city.

Police said just a handful of protesters were responsible for the violence, with most opting for peaceful demonstrations. ( Reuters: Pawel Kopczynski )

The protests marred a gathering that German Chancellor Angela Merkel had hoped would demonstrate her country's unshakeable commitment to freedom of speech and assembly.

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"I have every understanding for peaceful demonstrations but violent demonstrations put human lives in danger," she said.

Participants in the G20 meeting praised the work of police in keeping the event safe but said they had never seen protesters closer to such a summit than in Hamburg.

In the touristy Pferdemarkt area, known for its trendy cafes and nightlife, activists faced off against police in riot gear who were unable to put out fires, with billowing thick smoke dramatically reducing visibility.

In the nearby Schanzenviertel, looters plundered a supermarket. Nearby, a cash machine was burned out. Several police helicopters patrolled overhead.

The radical Black Bloc movement, which wants to overthrow capitalism, had some success in disrupting the leaders' discussions despite the ring of security around them.

Sorry, this video has expired Protesters clashed with police, leaving almost 200 officers injured

German Finance Minister Wolfgang Schaeuble cancelled an appearance in downtown Hamburg on Friday morning due to security concerns.

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Police also declined to clear US first lady Melania Trump's motorcade to leave her hotel for a tour of the city's historic harbour, her spokeswoman said.

Marine police units intercepted 22 divers from the environmental pressure group Greenpeace who had also been trying to reach the concert hall, police said.

Police said they used water cannon to disperse protesters who broke into a riverside metro station that had earlier been sealed off.

Police Chief Ralf Martin Meyer told reporters 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.

Almost 200 police were injured in the clashes. ( Reuters: Hannibal Hanschke )

In the violence, which first erupted during marches on Thursday (local time), 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 hovering police helicopters was nearly struck by a rocket flare, police said.

On Wednesday, "zombie" protesters descended on Hamburg for a protest of a different kind.

The performance protest attracted about 1,000 demonstrators.

About 1,000 protesters dressed as zombies for a protest on Wednesday. ( Reuters: Hannibal Hgmailanscke )

Reuters