Anti-capitalist protesters have clashed with police as the G20 summit gets under way in Hamburg, Germany.
Police used tear gas and water cannons against the crowds, which burned cars and held signs saying, "Smash G20!"
Ahead of the meeting, United States president Donald Trump spoke before a large crowd in Warsaw, declaring Western civilisation is under threat from terrorism and bureaucracy.
"The defence of the West ultimately rests not only on means but also on the will of its people to prevail and be successful and get what you have to have. The fundamental question of our time is whether the West has the will to survive. Do we have the confidence in our values to defend them at any cost?"
Mr Trump also addressed the issue of North Korea before the crowd, hinting military action might be an option.
"As far as North Korea is concerned, I don't know, we'll see what happens. I don't like to talk about what I have planned, but I have some pretty severe things that we're thinking about. That doesn't mean we're going to do them. I don't draw red lines."
Arriving in Hamburg, Australia's prime minister, Malcolm Turnbull, has suggested economic sanctions should be the path forward for controlling North Korea.
"We are a party to and, of course, support the sanctions imposed by the United Nations. And we also have autonomous sanctions of our own against North Korean entities and individuals. But we will work cooperatively, through the United Nations, and taking measured steps autonomously, but focused on bringing the economic pressure to bear on North Korea to bring that regime to its senses without conflict."
In what is set to be a politically charged G20 summit, the world leaders will discuss their responses to terrorism, the environment and globalisation.
As the host of this year's G20, German chancellor Angela Merkel admits there are differences between the leaders at the moment, but she says she is committed to cooperation.
"We are convinced that, if we address the big problems -- and we know that there are a lot of them in the world at the moment -- and if we make every effort to find solutions, that everyone can benefit from this, that it is a so-called win-win situation."
Germany's finance minister, Wolfgang Schaeuble, has also emphasised the importance of the United States working together with other world leaders.
"The strongest power of the world, economically and politically, if the strongest power wants to be even stronger, this power must lead the world. If the strongest power is saying, 'Only America,' it doesn't remain the strongest power. It becomes more ... more ... less important."
President Trump is scheduled to meet with Russian president Vladimir Putin tonight.
However, their meeting comes after Mr Trump said yesterday Russia may have interfered in the 2016 US election.
"I think it could very well have been Russia, but I think it could well have been other countries, and I won't be specific."
The G20 Summit will run for the next two days.
