ACTIVISTS, POLICE clash as protests start.

HAMBURG, Germany — German security forces wearing riot gear clashed with protesters on the eve of the Group of 20 summit in Hamburg, using water cannons and pepper spray to clear an anti-capitalist march in which a militant group with anarchist sympathies had a prominent presence.

The skirmish followed an hourlong standoff adjacent to Hamburg’s harbor, where protesters were attempting to move from a public square toward the downtown conference center where Germany’s chancellor, Angela Merkel, is hosting foreign leaders, including President Donald Trump, in a two-day summit that begins today.

When police attempted to separate “black bloc” activists from the roughly 12,000 people who had assembled to protest inequality and economic greed, authorities met a hail of rocks and bottles.

As police rushed the group, some of the protesters fled. But a phalanx of activists dressed in dark clothes, with their faces concealed, held their ground. They carried signs that condemned state powers and declared, “Welcome to hell.”

Armored vehicles spewing powerful bursts of water rolled toward the protesters. Smoke bombs detonated in the crowd.

Police made some arrests but said they did not have a final tally Thursday evening. They said 15 officers were injured, two of whom were hospitalized. A furniture store and a bank were damaged, police said.

Medics could be seen treating the injuries of protesters on the sidelines of the demonstration.

After police broke up the throng of anti-state militants clad in black, a lively crowd remained, resisting calls to disband as they chanted anti-capitalist slogans. The showdown persisted as the sun set in the northern German port city.

The street marches planned for the summit — emulating the forceful dissent seen at past G-20 gatherings — cover a range of concerns, including calls for environmental protection, denunciations of ethnic nationalism and opposition to free trade.

But the Hamburg protests have gained added momentum as a stand against Trump and his brand of “America First” populism.

An estimated 100,000 protesters were expected to converge on the old merchant city during the summit.

Meanwhile, 20,000 officers were being deployed at about 30 registered demonstrations, in the largest police operation in Hamburg’s history. Forty-five water cannons were available to disperse crowds. A no-fly zone was in place over parts 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.

Before Thursday’s protests began, officials raised fears that they could turn violent. But the gathering at first resembled an open-air concert, with bands from around the world performing. People shared potato stew and passed around art materials for posters. One sign announced support for the Black Lives Matter movement.

Sebastian Keller, 35, said he wanted to highlight how politicians serve only economic interests, not average people.

“I’m not anti-government, but something has to change so human beings get to enjoy a little bit of the wealth,” said Keller, who grew up in East Germany and was 8 years old when the country was reunified. “Ever since,” he said, “Germany has become obsessed with capitalism.”

Protests were expected to continue today and Saturday, stoked by the presence of contentious foreign leaders, including Russian President Vladimir Putin and Turkish President Recep Tayyip Erdogan.

Erdogan’s presence pits Turkish nationalists against Kurds in a country with the largest Turkish community outside Turkey. The German government has barred 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 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 spokesman for the Interventionist Left, a radical leftist group in Germany and Austria.