The agenda includes trade, women and climate change.
During the two-day gathering of leaders from the Group of 20, the meeting’s official themes will include global economic risks, trade disputes, job growth and investment, innovation and artificial intelligence, and women in the workplace.
Several countries are also hoping to emphasize collective action against climate change, which could become a point of contention. Japan is emphasizing the problem of plastic litter in the world’s oceans and seas, and President Emmanuel Macron of France, in a direct challenge to President Trump, has threatened not to sign any official joint statement out of the meeting that does not include an affirmation of the 2015 Paris climate agreement.
But Mr. Trump has called climate change a hoax, disputed its causes and vowed to withdraw the United States from the agreement.
In the hours before and after leaving for the summit, Mr. Trump lashed out at three nations whose leaders are attending — Germany, India and the host, Japan. He is scheduled to meet with those countries’ leaders on Friday.
The Group of 20, which is meeting in Osaka, Japan, this year, is an organization of finance ministers and central bank governors from 19 countries and the European Union. In addition to the United States and the bloc, its members include China, Japan, Germany, France, India, Brazil, Mexico, Canada, Saudi Arabia, Britain, South Korea, Turkey, Italy, Russia, South Africa, Indonesia, Argentina and Australia.
President Trump will talk with Presidents Putin and Xi.
On the sidelines of the summit meeting, Mr. Trump is expected to meet several leaders. But all eyes will be on two of his conversations, in particular — with President Vladimir V. Putin of Russia, on Friday, and President Xi Jinping of China, on Saturday.
Mr. Trump has expressed the hope of refreshing relations with Mr. Putin. When the two meet, it will be the first time since the release of the report by the special counsel, Robert S. Mueller III, on Russia’s meddling in the 2016 election in the United States.
The presidents had planned to meet last year, but Mr. Trump called their meeting off at the last minute, citing Russia’s seizure of three Ukrainian ships and detention of two dozen Ukrainian sailors, who remain in Russian custody. At Mr. Trump and Mr. Putin’s last formal session, in Helsinki, Finland, last year, Mr. Trump drew criticism and scorn for giving Mr. Putin’s denial of election interference equal weight to the contrary conclusion of American intelligence agencies.
Hanging over Mr. Trump’s meeting with Mr. Xi is the bitter trade standoff between the two countries; efforts to reach a deal fell apart this year. Economic, trade and security disputes have led to mutual tariffs that have put world markets on edge.
Mr. Trump is expected to meet with at least nine leaders directly. On Friday, he has scheduled talks with Prime Minister Shinzo Abe of Japan, Prime Minister Narendra Modi of India, Chancellor Angela Merkel of Germany, Mr. Putin and President Jair Bolsonaro of Brazil. On Saturday, he is to meet with Crown Prince Mohammed bin Salman of Saudi Arabia, Mr. Xi and President Recep Tayyip Erdogan of Turkey.
Osaka, Japan, is the host this year.
With so many world leaders in town, Osaka is going through the intensive security drill experienced by past hosts. According to Japanese news reports, 32,000 police officers will be guarding the city, where many streets will be closed and trash cans have been sealed to keep bombs from being placed. Even the city’s red-light district will be hanging discreet curtains and curbing illicit activities.
Japanese officials are campaigning this year to reduce the amount of plastic waste that flows into the oceans. Japan’s own record is mixed. Although it has been praised for improvements in trash collection and recycling, it is also a huge driver of plastic use, and the world’s second-largest generator of plastic waste per capita.
At a Group of 7 meeting last year, Japan and the United States did not sign on to a charter aimed at reducing marine waste. Japan and many Western nations also export their trash to poorer countries that may not have sophisticated recycling systems, resulting in more plastic waste that ends up in the sea.
A group of environmental ministers met this month in Japan and agreed on a set of voluntary measures to study the problem and clean up litter, which can destroy ecosystems, interfere with shipping navigation and clog beaches. Critics have said those measures do not go far enough.
Reporting was contributed by Michael Crowley and Motoko Rich from Osaka, Japan, and by Jane Perlez from Beijing.