Donald Trump and Vladimir Putin have met for the first time, shaking hands and holding face-to-face talks as the G20 summit in Hamburg began.
"It's an honour to be with you," said Mr Trump to Mr Putin, who responded: "I'm delighted to meet you."
The US and Russian leaders say they want to repair ties damaged by crises including Russia's alleged meddling in the US election.
Climate change and trade are set to dominate the two-day G20 meeting.
Violent clashes between protesters and police are taking place in the streets outside the venue, with dozens left injured.
US First Lady Melania Trump has been unable to leave her hotel in the German city because of the protests.
A huge police operation is trying to keep demonstrators - who are protesting against the presence of Mr Trump and Mr Putin, climate change and global wealth inequalities - well away from the summit venue, and water cannon have been deployed.
The G20 (Group of Twenty) is a summit for 19 countries, both developed and developing, plus the EU.
In her summit opening statement, German Chancellor Angela Merkel said: "We are all aware of the great global challenges.
"We know that time is short and therefore solutions very often can only be found if we are ready to compromise and work together without bending over backwards too much because, of course, we can express different views on some issues."