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Hamburg, Germany • Thousands of anti-capitalist protesters massed Thursday in a harborside square and engaged in small-scale skirmishes with police ahead of massive demonstrations planned as an angry counterpoint to a summit of the world's top industrialized powers.

The street marches and rallies — like at past Group of 20 gatherings — cover a range of issues including calls for environmental protections, denunciations of ethnic nationalism and opposition to free trade.

But the Hamburg protests have gained added momentum as a stand against President Donald Trump and his brand of "America First" populism. An estimated 100,000 protesters were expected to converge on the old merchant city during the G-20 summit, which begins Friday.

After gathering at the riverside plaza, protesters began marching through the city. Police called on demonstrators to remove masks, and some clashes were reported, with protesters tossing bottles toward police. Police responded by spraying protesters with water cannons as people recorded the scene on their cellphones.

Video footage showed masked protesters blocking streets with building materials from a construction site and setting small fires. Elsewhere, rows of helmeted riot police backed by water cannons and armored vehicles faced off against demonstrators chanting anti-capitalist slogans.

Police began removing some protesters who were blocking a crosswalk.

Authorities mounted the largest police operation in the city's history. At least 20,000 officers were being deployed at about 30 registered demonstrations. Forty-five water cannons will be on hand to disperse crowds. Some were used Tuesday evening to clear the streets of early protesters.

Officials raised fears the protests could turn violent. But hours before they were set to begin, the gathering resembled an open-air concert with bands from all over the world performing. People shared food and art materials for posters, as police formed an enormous cordon.

Moored a short distance away was a ship with a message plastered on its flank — "Keep global trade open" — at odds with the signs carried by protesters.

The protests draw on a tradition of left-wing activism in Germany's second-largest city and the birthplace of its chancellor, Angela Merkel. She is hosting a roster of foreign leaders — including divisive figures such as Trump, Russia's Vladimir Putin and Turkey's Reçep Tayyip Erdogan — at a downtown conference center and the lofty Elbphilharmonie concert hall, a crown jewel of the city.

A few miles away is the nerve center of left-wing German radicalism, Rote Flora, a former theater where activists have squatted for nearly three decades. Its members were some of those planning the anti-capitalist protest, which they dubbed "Welcome to Hell."

Security officials say the demonstration could draw as many as 8,000 members of the militant left, from Germany and beyond. Among its participants will be "black bloc" demonstrators with anarchist sympathies who wear dark clothes and cover their faces. Authorities said their concerns mounted following the discovery of materials used to prepare molotov cocktails, along with knives, slingshots and baseball bats.

A spokesman for Rote Flora, Andreas Blechschmidt, who registered the demonstration, said his hope is for a peaceful protest. But he promised self-defense "if the police attack us," saying, "Violence can be a productive form of protest."

A no-fly zone was in place over portions of the city.

"No demonstrator can decide whether or where heads of state and government meet in Germany on the chancellor's invitation," said Thomas de Maiziere, the German interior minister.

At stake are questions about security, free expression and democratic assembly — newly relevant alongside a summit that, although traditionally devoted to economics, may also showcase different approaches to human rights and the rule of law. Merkel, who is chairing the summit, said she will highlight climate, free trade and the shared obligation to assist refugees.

Her critics say her policies are part of the problem.

"This week is about Angela Merkel's austerity policy going global via G-20," said Jan van Aken, a member of the German Parliament representing the far-left Die Linke party.

He criticized the German government for seeking to stamp out protest, saying its approach was autocratic and would "make Erdogan, Putin and Trump feel at home here."

The government is sensitive to this point.

"The main issue is that the summit is again, after Brisbane, in a democracy," said Wolfgang Schmidt, a Hamburg politician involved in summit preparations.

Summits in Turkey and China followed the 2014 meeting in Australia. "You want to make sure that protest and dissenting views are heard, but you also need to maintain security, and with 42 highly protected heads of state and finance and foreign ministers, it's a challenge."

Tensions simmered over the weekend and into the week as police used force to remove activists attempting to stay overnight on public land. The courts have said that camps are a protected form of political protest but that authorities may prohibit certain forms of overnight assembly.

Erdogan's presence pits Turkish nationalists against Kurds, in a country with the largest Turkish community outside Turkey. The German government has disallowed Erdogan from addressing his supporters at the summit.

Yavuz Fersoglu, a spokesman for an umbrella organization of Kurdish groups in Germany, said Kurds are joining hands with anti-globalization groups for a major march on Saturday, which organizers say will draw about 100,000 people.

Trump is a particular flash point.

Planning for protests began before his November victory, but "it became clear after his election that the action would have to be much bigger," said Emily Laquer, a spokeswoman for Interventionistischen Linken, a radical left-wing group in Germany and Austria.

Local businesses were preparing for an unpredictable several days.

Richard Canning, the manager of a bar on a cobblestone street near the philharmonic that he said withstood much of the bombing during the Second World War, planned to close on Friday and Saturday out of concern for the safety of his staff.

He said he was sorry to lose business but happy to see Germany take on the difficult role of hosting international negotiations.

"I think that Germany is seen to be one of the major powers in Europe, and rightly so, because since the Second World War it has been building bridges, so I'm happy it's holding itself up to the world," Canning said.

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The Washington Post's Stephanie Kirchner contributed to this report.