Riot police have fired water cannon and tear gas at protesters who hurled bottles and bricks ahead of the G20 summit.
At least 76 police officers were injured in clashes with the demonstrators on the streets of Hamburg yesterday evening, as world leaders were arriving for the talks.
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The demonstration was dubbed 'Welcome to Hell' by anti-capitalist groups.
Police said the trouble involved around 1,000 people, but as many as 100,000 are expected to descend on the city to take part in protests during the two-day summit, which starts in earnest today.
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Sky's Alex Rossi, at the scene, said the violence - near Hamburg's fish market - was expected and that police had been preparing for trouble as demonstrators attempted to march through the city.
He said a few had been carried off with what appeared to be "quite serious" injuries.
"When we arrived earlier the atmosphere was peaceful and calm, people drinking and enjoying the sun, listening to speeches," said Rossi.
"Within that group there were people who looked like they had come here to cause trouble.
"As it got to early evening we saw a number of people coming down dressed in black, some of them masked with handkerchiefs across their faces."
The trouble died down after several hours and the situation became calmer, Rossi added.
However, there were reports of scuffles continuing throughout the night.
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Up to 20,000 police are on hand for the Hamburg summit given the history of violent protests at previous gatherings.
The G20 countries - whose members represent about 80% of the world's population - are meeting with their usual broad remit of making the world a more stable and prosperous place.
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