Organisers had cancelled the march following the violence, police said, but many demonstrators remained on the streets.
At least one person appeared to have been seriously hurt and was carried away covered by a foil blanket.
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World leaders including US President Donald Trump are in Hamburg for talks on a wide range of issues.
The clashes began when police charged a group of anti-capitalist demonstrators at the march attended by thousands carrying banners with slogans such as “Welcome to hell” and “Smash G20 “They fired water cannon and pepper spray at masked protesters, who hurled bottles, stones and flares at police.
Medics were seen treating several people. Other protesters built makeshift barricades on streets. At least one vehicle had been set alight and businesses had been damaged, police said.
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