World leaders meet to discuss urgent global issues in Hamburg, Germany.

World leaders pressure the U.S. President Trump to compromise on climate and trade.

In a joint message issued as the leaders gathered in a vast convention center in Hamburg, Brazil, Russia, India and China - the so-called BRICS countries - called on the G20 to push for implementation of the Paris climate deal despite Trump's decision last month to pull the United States out of it.

"The Paris agreement on climate change is an important consensus that doesn't come easily and must not be given up easily," said Chinese President Xi Jinping.

British Prime Minister Theresa May said G20 leaders would urge Trump to reconsider his decision on Paris. "We are not renegotiating the Paris agreement, that stays, but I want to see the U.S. looking for ways to rejoin it," she told the BBC.

The meeting comes at a time of major shifts in the global geo-political landscape, with Trump's "America First" policies pushing Europe and China closer together.

Trump will meet President Vladimir Putin for the first time on Friday afternoon, an encounter that will be intensely scrutinized following allegations by U.S. intelligence agencies that Moscow meddled in the U.S. election to help Trump win.

The summit also brings together Trump and Xi at a time when Washington is raising pressure on Beijing to rein in North Korea and threatening the Chinese with punitive trade measures.

Amid the big egos and seemingly intractable conflicts, the host, German Chancellor Angela Merkel, faces the tough task of steering leaders towards a consensus on trade, climate and migration - all issues that have become more contentious since Trump entered the White House half a year ago.

She faces an election in a little more than two months and cannot appear to cave in to Trump, who is deeply unpopular in Germany. Nor will she be keen for an open confrontation that could deepen tensions with Washington.

After a night of clashes with police, groups of anti-capitalist protesters sat on the main intersections in Hamburg, blocking streets and bridges leading to the summit venue in the city center as well as a road used by container trucks at Hamburg Port.

The blockades forced German Finance Minister Wolfgang Schaeuble to cancel a discussion with pupils at a school in the city. Police have used water cannon to clear roads that G20 officials are using. About 30 protesters have been arrested and more than 100 police officers injured.

Merkel exchanged friendly greetings with Trump and Putin as they entered the summit site, with Trump punching the air playfully with his fist.

The handshake between Merkel and Turkish President Tayyip Erdogan, who have been engaged in a war of words over human rights and access to an air base in Turkey where German troops are stationed, was noticeably less warm.

The bilateral meeting between Trump and Putin is scheduled to start just 15 minutes after G20 leaders are to begin their discussion on climate issues, a scheduling conflict that did not go unnoticed by the German hosts.

On the policy front, European sources said that so-called sherpas who negotiate the wording of the final G20 communique that is later signed off by the leaders, were converging on a formulation on climate that is based on draft conclusions reported by Reuters earlier this week.

The draft acknowledges U.S. isolation on the Paris climate accord and calls it "irreversible". Sources said Washington was pushing for a mention of fossil fuels as an alternative to cleaner energy sources, language that Europe was resisting.

On trade, sources said that Washington was backtracking on language condemning protectionism that Trump agreed to at a Group of Seven meeting in Sicily in late May.

"We firmly support a rules-based, transparent, non-discriminatory, open and inclusive multilateral trading system, implementation and enforcement of existing WTO rules and commitments and oppose protectionism," the BRICS countries said.

Hanging over the trade discussions is a threat by Washington to use a Cold War-era law to restrict steel imports based on national security concerns, a step that would hit the Chinese as well as partners in Europe.

European Commission President Jean-Claude Juncker said on Friday morning in Hamburg that the EU would respond "immediately and adequately" if the U.S. took action on steel.

After sessions on terrorism, the global economy and climate on Friday, the leaders will be joined by their spouses for dinner at the Elbphilharmonie, a striking new glass concert hall perched atop an old warehouse building overlooking the Elbe River.