Police and protesters have clashed in the German city of Hamburg where the G20 is due to start on Friday.
The violence began when police charged masked demonstrators at a “Welcome to hell” march attended by 12,000 people.
At least 15 police were hurt and three were being treated in hospital, police said. There were no figures for the number of demonstrators injured.
World leaders including US President Donald Trump are in Hamburg for talks on a wide range of issues.
Police fired water cannon and pepper spray at masked protesters, who hurled bottles, stones and flares.
Organisers cancelled the march where the first clashes took place, but protesters remained on the streets and police said violence spread to other areas of the city.
Protesters built makeshift barricades, set vehicles alight, damaged businesses and repeatedly shone a laser at a police helicopter to dazzle its pilot, police said.
Medics were seen treating several people. At least one person appeared to have been seriously hurt and was carried away covered by a foil blanket.
Before the march, police had warned of possible violence and said they had confiscated a number of homemade weapons.
Some 20,000 police have been deployed in Hamburg for the summit, and security cordons have been erected to prevent protesters reaching the venues. Up to 100,000 protesters are expected in Hamburg during Friday and Saturday.