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

 

Fears over nuclear-armed North Korea, which just successfully tested an intercontinental ballistic missile (ICBM), cast another long 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 heads of the Group of 20 big industrialised and emerging economies against anti-capitalist protesters are greeting them with the combative slogan "G20 -- Welcome to Hell".

 

But trouble is also brewing 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 had vowed North Korea's goal of developing a nuclear weapon that can reach the US "won't happen" and has repeatedly pressed China to rein in its truculent neighbour.

 

His counterparts are bracing for fresh surprises after Trump stunned the world by pulling out of the 2015 Paris climate pact, questioned long-standing NATO allegiances and dismissed free-trade principles.

 

"There is a danger that the summit will lead to polarisation between the US and other countries" on climate change and other issues, warned Oxford Analytics economist Adam Slater.

 

Trade wars loom as Trump has demanded Germany and China reduce their huge surpluses and his administration has threatened other countries with 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.

 

- 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.

 

Chinese President Xi Jinping, who meets German Chancellor Angela Merkel in Berlin on Wednesday, will hold talks with regional rival Japan's Prime Minister Shinzo Abe, at a time when both increasingly worry about the threat posed by Pyongyang.

 

Also looming over the summit will be the bloody conflict in Syria and the frozen one in Ukraine -- both involving Russia -- as well as the struggle for Mideast dominance between Saudi Arabia and Iran.

 

Then there is Turkey's President Recep Tayyip Erdogan, whose crackdown on alleged coup plotters after a 2016 putsch attempt has poisoned relations with Germany.

 

Trump, before the summit, 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 debt crisis.

 

The US president will on Thursday meet Merkel, the longest-serving leader and, aside from Britain's Theresa May, the only woman in the G20 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."

 

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.

 

The protesters are a diverse group of environmentalists, peace and anti-poverty activists, united in the belief that the world's elite guarded by riot police is failing to solve the pressing global problems.

 

"We think that they all deserve our protest and our resistance from the streets," said one activist, Georg Ismael, 25.

 

"We are demonstrating against the G20 because we don't think that they represent the interests of humanity."