Thousands of people have turned up for the first major anti-capitalist protest march ahead of the G20 summit in Germany.
The early part of the day was largely peaceful but later clashes left more than 70 police officers and an unknown number of protesters injured.
20,000 security forces have been deployed during the two-day meeting of world leaders which begins on July 7.
People of all ages, from babies to pensioners, joined in the morning procession in the city of Hamburg.
Many tourists also took part.
Demonstrators say the G20 group of the world's leading economies has failed to solve many of the issues threatening global peace and the environment.
There was a huge police presence around the city's market square where the main rally was held, with rows of vans, water cannon and officers in heavy riot gear positioned in all directions.
Demonstrator Dirk Mirkow said he was there because he wanted to protest wealth inequality and the presence of the leaders in Hamburg, "Donald Trump denies climate change. And because the richest eight men own as much as a large percentage of the world's population."
The first clashes broke out when another demonstration kicked off in the city center on Thursday afternoon.
The Welcome to Hell organizers had promised "to turn Hamburg into a location and an exclamation mark of resistance against old and new authorities of capitalism."
Protesters tried to march towards a footbridge towards the summit venue but lines of riot police blocked their path.
Officers said the unrest then spread to other areas of the city.
A Reuters witness reported seeing one demonstrator with blood on his face being treated.
Medics were seen attending to several other people.
At least one person appeared to have been seriously hurt and was carried away covered by a foil blanket.
Reports say one person has been arrested.
Up to 100,000 protesters are expected in Hamburg during Friday and Saturday.
The clashes took place as the German Chancellor Angela Merkel met the U.S. President Donald Trump for a bilateral meeting before the summit.
Merkel has taken a high-risk gamble by choosing to hold the talks in the northern port city, partly to show the world that big protests are tolerated in a healthy democracy.
Before meeting Trump, she struck a consensual tone, holding out hope for agreement on the divisive issue of climate policy and promising to broker compromises.
She pledged to represent German and European interests at the summit, but added, "On the other hand, as hosts we - and I - will do all we can to find compromises."
Trump faces a testy confrontation at the summit with leaders of the other big Group of 20 economies after deciding last month to pull the United States out of the 2015 Paris climate deal.