By Joseph Nasr and Sabine Siebold

HAMBURG (Reuters) - Fires burned across downtown Hamburg on Friday evening for the second day of clashes between anti-capitalist protesters and police while leaders of the world's 20 most powerful countries listened to Beethoven's 9th Symphony and dined in style.

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 and lorries, looted retail stores, lit off firecrackers and blasted music from Jimi Hendrix in a vain bid to drown out the classical music playing at the Elbphilarmonie concert hall across the Elbe River.

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

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.

"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.

DISRUPTIONS

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.

German Finance Minister Wolfgang Schaeuble canceled an appearance in downtown Hamburg on Friday morning due to security concerns. Police also declined to clear U.S. first lady Melania Trump's motorcade to leave her hotel for a tour of the city's historic harbor, her spokeswoman said.

"Thinking of those hurt in #Hamburg protests. Hope everyone stay safe!" the U.S. president's wife tweeted afterward.

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 cannons 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.

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.

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 hovering police helicopters 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.

(Reporting by Joseph Nasr, Andrea Shalal, Thomas Escritt and Klaus Lauer; Editing by Gareth Jones and Bill Trott)