• Mr. Trump wants the United States to wield its economic dominance to dictate the rules of global trade, but other countries seem unwilling to follow. As if to make that point, the European Union and Japan agreed on Thursday to the outlines of a trade deal that would diminish opportunities for American companies.
• The United States, Japan and South Korea — but not China — called for a United Nations Security Council resolution condemning North Korea’s long-range missile launch, and for accelerated sanctions to pressure Pyongyang.
All eyes were on Trump and Putin.
Mr. Trump and Mr. Putin met on Friday, and while their face-to-face encounter was not officially the main event at the G-20, for many, it might as well have been. Scores of journalists shouted questions as the two leaders sat side by side for photographs before the actual meeting.
“President Putin and I have been discussing various things, and I think it’s going very well,” Mr. Trump said. “We look forward to a lot of very positive happenings for Russia and for the United States, and for everyone concerned.”
Mr. Putin said he was happy to have the chance to meet Mr. Trump in person. “We spoke over the phone,” he said, “but phone conversations are never enough, definitely.”
He added: “I hope that, as you have said, our meetings will yield positive results.”
Only six people attended the meeting: Mr. Trump and his secretary of state, Rex W. Tillerson; Mr. Putin and his foreign minister, Sergey V. Lavrov; and two interpreters.
The Russians had agitated to include several more staff members in the meeting, but Mr. Trump’s team had insisted that the meeting be kept small to avoid leaks and competing accounts later, according to an administration official with direct knowledge of the carefully choreographed meeting, who spoke on the condition of anonymity because of the sensitivity around the matter.
Even a brief handshake earlier in the day, before the proceedings officially opened, was the subject of enormous scrutiny.
The leaders have little obvious common ground.
In Warsaw, where he gave a speech on Thursday before flying to Hamburg, Mr. Trump delivered a mixed message on Russia. He urged it to “cease its destabilizing activities in Ukraine,” and denounced its support for “hostile regimes,” including Iran and Syria, and offered unqualified support for the collective defense principle of NATO.
At the same time, he broke with American intelligence agencies by saying he was not entirely convinced that Russia was solely responsible for interference in the 2016 election.
In Moscow, there was a sense that Mr. Putin will be able to outwit and outmaneuver the American leader and come out on top. “It is a win-win situation for Putin,” said Andrei V. Kolesnikov, a political analyst at the Carnegie Moscow Center, though it will not be all smooth sailing for Mr. Putin.
The two leaders find themselves on opposite sides of several important issues, including climate change and Western sanctions imposed on Russia after its annexation of Crimea. The Kremlin is also rankled by a missile defense system that the United States is building in Eastern Europe.
They might find some common ground on counterterrorism. But in Syria, Moscow is backing President Bashar al-Assad, while Washington still wants to see him step down.
Protesters got an early start. So did the police.
Hamburg awoke on Friday to the buzz of helicopters and the wailing of sirens, as police officers rushed to keep up with protesters who had gathered at the city’s major intersections in an effort to block the routes G-20 leaders were to take to the Messehallen Convention Center, the site of the meeting.
Protesters burned several vehicles and set fire to trash hauling bins overnight, and columns of smoke could be seen rising over the city again early Friday. Taxi drivers were avoiding the city center, some in protest, others to protect their vehicles.
“We remind you that gatherings in the transit corridors will not be tolerated,” the police said on Twitter. The authorities used water cannons to stop the protesters from advancing.
The police presence was enormous near the convention center. In black riot gear including helmets, padding and sometimes face masks, the police stood in small groups in a quiet face-off with civilians who might or might not have been demonstrators. The authorities have said that 20,000 police officers would be deployed.
Many streets in and around the city center — which is famous for its extreme left-wing scene — were blocked to ordinary traffic, though nearby public transport stations were open, albeit with increased security patrols.
Demonstrations Thursday night turned violent after the police moved in to separate a group wearing balaclavas and masks — which German law forbids during public protests — in a section of the 12,000 people who had filled the streets outside the security perimeter. — Melissa Eddy and Steven Erlanger
The ‘family’ posed for a group photo.
After finally corralling the leaders to stand for the official “family photograph,” and persuading them to stop schmoozing with one another and take their seats, Ms. Merkel delivered a short and relatively anodyne opening statement.
The German chancellor said she hoped that the summit meeting would “contribute to allaying” the “fears, needs and anxieties” of the world’s peoples. “We all know the great global challenges,” she said, “and time is pressing.”
Those in the room represent two-thirds of the world’s population, four-fifths of the world’s gross domestic product and three-quarters of the world’s trade, she said, and the rest of the world expected results.
Mr. Trump was on the side of the group photo, where leaders were mostly arrayed by seniority. Ms. Merkel stood at the center, flanked by the leaders of China, which hosted last year’s G-20 summit meeting, and Argentina, which will host next year’s.
Ms. Merkel noted that the symbol of this meeting — a naval reef knot — was intended to show that the world is interconnected.
“The more you pull on it, the better it holds,” she said. — Steven Erlanger and Alison Smale
What’s on everyone’s mind? Podesta, Trump says.
On a day in which he might — or might not — confront Mr. Putin on Russia’s attempts to sway the 2016 election, Mr. Trump decided to mount a diversionary attack against an American adversary.
“Everyone here is talking about why John Podesta refused to give the DNC server to the FBI and the CIA. Disgraceful!” Mr. Trump wrote in a tweet sandwiched between polite happy-to-meet tweets about Mr. Putin, Ms. Merkel, and Prime Minister Shinzo Abe of Japan.
Mr. Trump’s tweet was off the mark on three counts:
• Mr. Podesta was Hillary Clinton’s campaign chairman at the time and had no authority to turn over anything, much less someone else’s emails, to the F.B.I. and C.I.A.
• Mr. Podesta — whose own emails were targeted by hackers — fully cooperated with law enforcement agencies.
• The Democratic National Committee, which was leery of the F.B.I. because of its inquiry into Mrs. Clinton’s use of a private email server, did deny investigators access to their servers. But it gave the bureau information that later pointed to Moscow’s interference in the election, according to congressional testimony from James B. Comey, the former F.B.I. director. — Glenn Thrush
Protests force Melania Trump to stay at her guesthouse.
The first lady, Melania Trump, scrapped her public schedule on Friday because the local police, who are dealing with protests, would not allow her to leave the guesthouse where she and Mr. Trump are staying because of security concerns, her spokeswoman said.
Mrs. Trump, who was to attend a boat tour, a luncheon and a visit to a climate-control facility with the spouses of other G-20 leaders, instead stayed cloistered in the residence, away from the mayhem. “She was very much looking forward to the day,” said the spokeswoman, Stephanie Grisham.
While other world leaders and their delegations stayed in hotels near the site of the summit meeting, the Trumps are staying at a guesthouse in another area.
Protesters poured through the streets near the convention center on Friday carrying banners with anti-globalization messages and some anti-Trump slogans. “Refugees welcome,” one sign said, while others read “No G-20” and other still insulted Mr. Trump in colorful terms.
Students cut class to take a stand in Hamburg.
Walking along the river in Hamburg, hundreds of students skipped their classes to take part in a peaceful demonstration, chanting, “One solution, revolution” and protesting against education systems in Germany and elsewhere.
“I’m here because I oppose how education in Germany is structured, that they’re training us to be workers and not thinkers,” said Hendro Myrow, 18, a student in Hamburg.
Lisa Müller, who helped organize the march, said its leaders had tried to draw as many students as possible. “The whole system of capitalism is the problem, because you need losers and winners, and that goes on at school as well,” she said. “It’s about learning one thing but not what you want to learn.”
Groups of riot police officers observed the march as it progressed. Police trucks blocked several streets, as various protest leaders took turns with a loudspeaker to criticize the education system, capitalism and the G-20. — David Shimer
E.U. warns U.S. against protectionism.
Cecilia Malmstrom, the European trade commissioner, warned the Trump administration on Friday against taking measures that would penalize European steel exporters.
Ms. Malmstrom declined to say what measures Europe might take in retaliation to United States action, or what specific exports might face higher tariffs, but she acknowledged that Brussels and several European Union member states had expressed concern to the United States that it might take “possible protectionist measures.”
“Barriers on steel imports would be very bad for the E.U., as European businesses and workers could be affected very heavily and jobs would be threatened,” Ms. Malmstrom said.
“If global trade rules are not upheld, the E.U. will retaliate,” she added. “But we don’t think this is the right way to go, as you cannot fight protectionism with protectionism.” — James Kanter