Police and protesters have clashed in the German city of Hamburg where the G20 is due to start on Friday.
Police fired water cannon at a march attended by thousands carrying banners with slogans such as "Welcome to hell" and "Smash G20".
World leaders including US President Donald Trump are in Hamburg for talks on a wide range of issues.
They face their own disagreements, including over climate change and trade.
TV pictures showed masked black-clad anti-capitalist activists hurling bottles, stones and other objects at police in riot gear. Medics were seen treating several people. Other protesters were building makeshift barricades on streets.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.
Image copyrightREUTERS
Image copyrightREUTERS
Last week Mrs Merkel said the G20 would focus on the Paris climate deal - which the US has withdrawn from.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 and 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.
Russia's waiting game - BBC's Sarah Rainsford, Moscow
Mr Trump referred to Russia's "destabilising" behaviour twice in one day in Poland. But the Kremlin spokesman has shrugged that off, saying simply that Moscow "does not agree". It's all part of the wait-and-see approach here.
Russia once had great hopes that Donald Trump could rescue relations from the pit into which they were plunged after it annexed Crimea from Ukraine. Almost six months into the Trump presidency, there may be increasing pessimism.
But the Kremlin is calling Mr Trump's meeting with Mr Putin on Friday an important chance to get acquainted. Perhaps it is betting that personal dynamics will help overcome policy differences.
After all, officials here insist that it is simply "Russophobia" in the US that has prevented President Trump "getting along" with Russia as he said he wanted.
They have certainly noted how in Poland he shied away from accusing Russia unequivocally of meddling in the US elections. Moscow has argued all along that there is no proof. In public at least, Mr Trump appeared to agree with that.
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.
Russian TV dismisses Poland visit - BBC Monitoring
NTV correspondent - "After the icy reception [Trump] was given in Europe in May what he needs now are comfortable and favourable surroundings, a picture along the lines of 'look at how they adore us here'."Ren TV presenter - Trump was keen to play on differences within Europe and help Poland "cobble together an Eastern European bloc opposed to EU leaders... Trump is only too happy to pour oil onto the fire of European discord."
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