HAMBURG, Germany — German security forces clad in riot gear clashed with protesters on the eve of the Group of 20 summit here, 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 a group of "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 the state and declared, "Welcome to hell."

Armored vehicles spewing powerful volleys 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 a large 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.

Police expressed concern that violence could escalate after nightfall. "We are skeptical as to whether this evening and tonight will remain peaceful," Hamburg police Chief Ralf Martin Meyer said on ZDF television.

The street marches planned for the summit cover a range of issues, 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 cannon were available to disperse crowds. A no-fly zone was in place over portions of the city.