Right Now: President Trump is meeting face to face with his Russian counterpart, Vladimir V. Putin, at the Group of 20 summit.

• Several high-profile issues are on the agenda of the main gathering, including climate change, global trade, North Korea, Syria and Ukraine. Leaders will also be meeting in smaller groups.

• Protests resumed on the streets of Hamburg, after 12,000 protesters converged for a demonstration in the city on Thursday night called “Welcome to Hell.” Both the police and demonstrators reported injuries, and cars were set on fire. As many as 100,000 protesters are expected to turn out.

• Mr. Trump was active on Twitter on Friday morning. He described the reaction to his speech in Poland and a meeting with Chancellor Angela Merkel of Germany as “great,” and then said “everyone here” is talking about John Podesta, Hillary Clinton’s campaign manager.

• 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.

All eyes on Trump and Putin as they meet in Hamburg.

Mr. Trump and Mr. Putin met on Friday, and while their face-to-face encounter is not officially the main event at the G-20, for many, it might as well be. Even a brief handshake earlier in the day, before the proceedings officially opened, was the subject of enormous scrutiny.

Mr. Trump delivered a mixed message on Russia while in Warsaw, issuing his sharpest criticism of Moscow since taking office. He called on Russia to “cease its destabilizing activities in Ukraine,” denounced its support for “hostile regimes,” including Iran and Syria, and offered unqualified support for the collective defense principle of NATO (something he had been unwilling to do during his first trip to Europe as president in May).

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 is 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 efforts, broadly speaking. But in Syria, Moscow is backing President Bashar al-Assad, while Washington still wants to see him step down.

Protesters get an early start, and the police are there to meet them.

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

Enough with the schmoozing: Merkel gets the summit underway.

After finally corralling the leaders to stand for the official 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, for one, is not on the same page as his counterparts from the European Union, China and Russia on many matters. Still, 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 in Germany? It’s John 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 the first lady to abandon plans and 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.

Mrs. Trump posted a message on Twitter on Friday expressing concern for those hurt in the demonstrations. — Julie Hirschfeld Davis

World leaders don’t have long to discuss climate change with Trump.

Mr. Trump’s meeting with Mr. Putin on Friday was likely to cut short a potentially uncomfortable discussion on climate change, a major element of the agenda on which he has broken with European allies.

The American and Russian leaders’ meeting was scheduled to begin just 15 minutes after the start of a G-20 working session on “Sustainable Growth, Climate, and Energy,” a discussion in which the president’s decision to withdraw from the Paris climate accord was likely to figure prominently.

If participants planned to lecture or otherwise challenge Mr. Trump on the matter, they would have had only 10 minutes to do so. — Julie Hirschfeld Davis

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