HAMBURG — For a thousand years, this port city has connected northern Europe to the far reaches of the globe.

Its international character will come into focus when world leaders gather here for the G-20 economic summit, which begins Friday. But its trademark openness will be tested as up to 100,000 protesters turn the old merchant city into a site of a global contest over capitalism, the environment and political and ethnic nationalism.

Their 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 President Trump, Russia’s Vladimir Putin and Turkey’s Recep Tayyip Erdogan — at a downtown conference center and the lofty Elbphilharmonie concert hall, a crown jewel of the city, which is among the country’s most affluent and yet burdened by an unemployment rate higher than the national average.

Diplomatic talks will unfold just a few kilometers from a nerve center of left-wing German radicalism, Rote Flora, a former theater where activists have squatted for nearly three decades. Its members are planning an anti-capitalist protest called “Welcome to Hell,” scheduled to begin Thursday evening as delegations arrive.

Warning of violence, 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.”

Authorities are promising the largest police operation in the city’s history. Twenty thousand officers will guard 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 protesters.

Swaths of the city near the central harbor will be cordoned off to counteract a summit blockade planned for Friday. A no-fly zone will be in place.

“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.

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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.

Issues motivating protest range from climate change to disarmament, from labor rights to lesbianism. And it won’t all be so serious. Musical performances will punctuate marches. A rave in the harbor was planned for Wednesday evening.

But the summit makes Hamburg a cauldron of global strife.

[Germany says Turkish bodyguards involved in D.C. skirmish ‘won’t set foot on German soil’]

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.

Stephanie Kirchner contributed to this report.

https://www.washingtonpost.com/world/europe/hamburg-braces-for-mass-protests-asg-20-makes-the-city-a-siteof-global-discord/2017/07/05/afafae46-6044-11e7-80a2-8c226031ac3f_story.html?utm_term=.1d158c319875