Update: 11.57am: Donald Trump and Vladimir Putin have met briefly ahead of their highly anticipated meeting on the sidelines of the G20 summit in Germany.
The two leaders had time to shake hands and exchange a few words upon arrival at the summit of industrialised and developing nations in Hamburg.
Their sit-down meeting, which may tackle a number of foreign policy issues from the conflict in Syria to Russia's provocations in Ukraine, will be overshadowed by investigations into whether Mr Trump's campaign co-ordinated with Moscow during last year's presidential election.
Mr Trump, during a speech in Warsaw on Thursday, urged Russia to "cease its destabilising activities in Ukraine and elsewhere and its support for hostile regimes - including Syria and Iran - and to instead join the community of responsible nations in our fight against common enemies and in defence of civilisation itself".
But much of the focus - in Washington and Moscow - will be on whether Mr Trump broaches the issue of Russia's meddling in the election.
Before the meeting on Friday, the US president tweeted that he was looking forward to the visit, saying there was "much to discuss".
Earlier: The G20 leaders' summit is getting under way in the German port city of Hamburg in the wake of clashes between police and protesters, with terrorism, global trade and climate change among the issues on the agenda.
The host, German Chancellor Angela Merkel, says she hopes to find "compromises and answers" on a range of issues at the two-day meeting of leading industrial and developing nations.
While there is little disagreement on fighting terrorism, prospects of finding common ground on climate change and trade look uncertain.
The meeting follows skirmishes on Thursday evening between police and protesters elsewhere in Germany's second-biggest city. Police said that at least 76 officers were hurt, one of whom had to be taken to a hospital with an eye injury after a firework exploded in front of him.
On Friday morning, dozens of protesters attempted to block cars from accessing the summit, being held at the trade fair grounds in downtown Hamburg, but they were quickly thwarted by police.
Further away in the city's Altona district, police said people set several parked cars alight and attacked a police station, though the situation quickly calmed down.
The city has boosted its police with reinforcements from around the country and has 20,000 officers on hand to patrol Hamburg's streets, skies and waterways.
The G20 comprises Argentina, Australia, Brazil, China, Germany, France, Britain, India, Indonesia, Italy, Japan, Canada, South Korea, Mexico, Russia, Saudi Arabia, South Africa, Turkey, the United States and the European Union.
Also attending the summit are the Netherlands, Norway, Spain, Guinea, Senegal, Singapore and Vietnam.