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.
The G20 leaders face their own disagreements, including over climate change and trade.
Mr Trump has already met German Chancellor Angela Merkel and the pair spent an hour talking about North Korea, the Middle East, the conflict in eastern Ukraine and G20 issues, a German government spokesman said.
Last week Mrs Merkel said the G20 would focus on the Paris climate deal – which the US has withdrawn from. But earlier she said that as the G20 host she would work to find compromises.
The summit will also see Mr Trump meet Russian President Vladimir Putin for the first time. The meeting will take place at 14:45 local time (13:45 GMT) and last for an hour, Russian media report.
Russia ‘destabilising’ Ukraine
Earlier in the day Mr Trump used a speech in the Polish capital Warsaw to call on Russia to stop “destabilising” Ukraine and other countries.
Russia should also end support for “hostile regimes” such as those in Syria and Iran and “join the community of responsible nations”, he said.
He urged Russia to join the “fight against common enemies and in defence of civilisation itself”.
The Kremlin rejected his comments.
The US leader also hailed Poland as an example of a country ready to defend Western freedoms.
Poland’s conservative government shares Mr Trump’s hostile view of immigration and strong sense of sovereignty.