An anti-G20 protest march in Germany by around 12,000 people was called off by organisers on Thursday after clashes between police and around 1,000 leftist demonstrators, police said.
"The march was just declared off by organisers," Hamburg police said on Twitter after the march quickly descended into chaos a day ahead of the start of the two-day G20 summit.
As several thousand demonstrators vowing to "Smash G20!" marched during the day, German Chancellor Angela Merkel welcomed arriving leaders of the world's top economic powers to the G20 summit set to discuss ways of "combating terror", free trade and efforts to combat climate change.
Police in Hamburg fired water cannon and pepper spray at protesters gathered to demonstrate against the G20 summit after a group of black-clad demonstrators threw stones, bottles and other objects at riot police.
Riot police in helmets charged to disperse a group of around 1,000 hard-left protestors wearing black hooded tops and masks.
Eyewitnesses saw at least one protester with blood on his face being treated. "Welcome to Hell" was the protesters' greeting for Trump and other world leaders arriving for the two-day meeting.
Germany's second city, hosting its largest-ever international meeting, has deployed some 20,000 police, equipped with riot gear, armoured vehicles, helicopters and surveillance drones.
A holding centre for detainees was set up with space for 400 people and detention judges were on standby.
Some 30 demonstrations were announced before and during the meeting, organised by anti-globalisation activists and environmentalists, trade unions, students and church groups.
Late on Thursday, police detained six people for throwing bottles at officers as about 11,000 protesters marched through Hamburg to techno music under the banner "I'd rather dance than G20".
Police also investigated a fire at a local Porsche dealership in which eight luxury vehicles went up in flames.
TRT World's correspondent Simon McGregor-Wood reports.