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 hour-long stand-off adjacent to Hamburg's harbour, where protesters were attempting to move from a public square towards the downtown conference centre where Germany's chancellor, Angela Merkel, is hosting foreign leaders, including President Donald Trump, in a two-day summit that began on Friday.

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."

Armoured vehicles spewing powerful volleys of water rolled towards the protesters. Smoke bombs detonated in the crowd.

Police made some arrests but said they did not have a final tally on Thursday evening. They said 15 officers were injured, two of whom were hospitalised. 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 sceptical 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 – emulating the forceful dissent seen at past G20 gatherings – 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 Mr Trump and his brand of "America First" populism. An estimated 100,000 protesters were expected to converge on the 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 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.

Before Thursday's protests began, officials had 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 Mr Keller, who grew up in East Germany and was eight years old when the country was reunified. "Ever since," he said, "Germany has become obsessed with capitalism."

The protests draw on a tradition of left-wing activism in Germany's second-largest city and the birthplace of its chancellor. A few miles away is a nerve centre of left-wing German radicalism, Rote Flora, a former theatre where activists have squatted for nearly three decades.

A spokesman for Rote Flora, Andreas Blechschmidt, who registered Thursday's demonstration, promised self-defence "if the police attack us." He added: "Violence can be a productive form of protest."

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

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

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

Mr 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 the Interventionist Left, 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 World War II, 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," Mr Canning said.