Berlin (AFP) - US President Donald Trump meets other world leaders at Germany's G20 summit from Friday, with conflicts looming over climate, trade and global security both inside and outside the heavily fortified venue.

Fears over nuclear-armed North Korea rose sharply after it successfully tested an intercontinental ballistic missile (ICBM), casting another shadow over the gathering which will bring the leaders of China, Japan and South Korea to the northern city of Hamburg.

Some 20,000 police will guard the leaders of the Group of 20 big industrialised and emerging economies as well as the UN, IMF, World Bank and other bodies, against anti-capitalist protesters who are greeting them with the combative slogan "G20 -- Welcome to Hell".

But trouble is equally expected at the conference table at a time when the West and Europe are deeply divided, the post-Cold War order is fraying and China and Russia are asserting themselves on the global stage.

All eyes will be on Trump, who stunned the world by pulling out of the 2015 Paris climate pact, questioned long-standing NATO allegiances and dismissed free-trade principles in favour of an "America First" stance.

On North Korea, he has vowed that the regime's goal of developing a nuclear weapon that can reach the US "won't happen" and on Monday angrily called on China to "put a heavy move on North Korea and end this nonsense once and for all!"

Trump plans to huddle with the leaders of Japan and South Korea to discuss the issue on Thursday evening.

He is also at the centre of heated disputes over global trade, having demanded that Germany and China reduce their huge surpluses and threatened punitive measures in battles over cars, steel and natural gas.

In the most anticipated moment of the G20, Trump will meet Russian President Vladimir Putin, the ex-KGB agent accused of having aided, with hackers and fake news, the surprise ascent of the property tycoon to the White House.

The moment they shake hands is sure to see "an Olympian level of macho posturing between these two leaders, who both understand the importance of symbolism and the perception of being tough," said Derek Chollet of the German Marshall Fund of the United States.

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- Global power club -

The year's biggest diplomatic event outside the UN will also provide a stage for other world leaders muscling for power and regional influence.

Meeting in Berlin on Wednesday, Chinese President Xi Jinping and German Chancellor Angela Merkel pledged close cooperation -- the diplomacy sweetened by China's loan of two panda bears to a Berlin zoo and a huge Airbus sale to China of aircraft worth $22.8 billion.

The leader of China's regional rival Japan, Prime Minister Shinzo Abe, is meanwhile set to announce Thursday plans for a broad trade deal with the EU at a Brussels summit, hailed as a victory for free trade that contrasts with Trump's decision to withdraw from a trans-Pacific regional agreement.

On the same day, Trump will meet leaders from ex-communist eastern European nations, including Hungary's authoritarian Viktor Orban -- threatening to deepen a new east-west split in the EU, which is facing Brexit and still recovering from divisions over the eurozone debt crisis.

The US president will then hold talks later Thursday with G20 host Merkel, the longest-serving leader and, aside from Britain's Theresa May, the only woman in the club.

- Street resistance -

Merkel, hailed by some as the "new leader of the free world", advocates an internationalist approach to global issues -- but her G20 motto, "Shaping an Interconnected World", contrasts sharply with Trump's go-it-alone approach.

The chancellor, a green energy champion who has allowed more than a million mostly Muslim refugees into Germany since 2015, said last week that "the differences are obvious and it would be dishonest to try to cover that up. That I won't do."

Merkel drove the point home in comments to weekly magazine Die Zeit, saying that Washington now "sees globalisation as a process that's not about achieving win-win outcomes but about winners and losers".

In Hamburg, she must walk a fine line between trying to build a 19-1 front against Trump on key issues and preventing even further damage to transatlantic ties, while seeking common ground for at least a watered-down final G20 communique.

On the streets, up to 100,000 protesters will march in some 30 rallies, creating a security headache in Germany's second-largest city. Police expect up to 8,000 leftwing radicals ready to use violence.

In a sign of what may come, police overnight clashed with hundreds of protesters as they cleared out a protest camp, using pepper spray and water cannon against activists on the streets.