HAMBURG, Germany—On Thursday night a crowd of thousands had just managed to advance roughly 300 meters along its intended route from the harbor area here toward the building where the G-20 summit will begin Friday morning, when a loud bang went off. Suddenly there emerged, from a truck emblazoned with the slogan “WELCOME TO HELL,” the head of a person wearing a black cap and neon green sunglasses who began to scream over a megaphone: “It happened as we expected! The police are attacking the demonstrators!“ and then another voice from the truck shouted: “Stay safe. Stay in groups.”

The demonstrators crawled up the walls of the street onto the river path, where the shrieking onlookers helped them. Police followed with water cannons, spraying into the crowds of people in scenes suggestive of the late 1990s and early Aughts, with the Battle of Seattle during the World Trade Organization conference and the deadly riots in Genoa when the G-7 convened there.

Some 76 police officers were injured, three of them hospitalized. A small group of angry demonstrators attacked police spokesman Timo Zill, who had to hide in an ambulance. It’s still unclear how many protesters were injured (I saw three people). According to Bild Zeitung, the apartment of the interior minister of Hamburg, Andy Grote, was attacked. Confrontations went on into the early hours of the morning. Bicycle stands and cars were set alight, windows smashed, and the water cannons deployed again.

Long before German Chancellor Angela Merkel welcomed world leaders like Donald Trump and Turkey’s Recep Tayyip Erdogan to Hamburg on Thursday, the streets of the city had filled with all kinds of creative protests, from all-night raves to zombie performance art to the Trump and Putin figurines being sold as chew toys for dogs.

For Merkel, “the safety issue is one of the most important things at this summit,” said Katharina Gnath, a specialist in European governance at the Bertelsmann Foundation. “She is very concerned that there could be some unreasonable crackdown like in Genoa or Seattle.”

But the Hamburg police chief had warned that the “Welcome to Hell” march would be “the demonstration on which all the violence-prone left-wing extremists will focus together; everyone who comes to the city with the goal of militant protest will take part in this gathering.” And the cops were having none of it.

Members of the hardcore left Schwarze Block (the black bloc) led the parade. They wore their standard uniform of black hoodies or raincoats and sunglasses. As they got into formation to a mixture of German hip-hop and Tricky, they peeked over their banners at the surrounding crowd of young people, some of whom were clutching plastic water bottles; others had glitter in their face.

“They are left-wing radicals. Their protest is against the state of the world... the unfairness,” Nora, a university student standing at the side of the road with her friend Caroline, told The Daily Beast. Both said they’d been to the May Day demonstrations in Berlin this year as well. The Schwarze Block makes a major appearance in this annual protest. It actually ran very smoothly this year, but Nora smirked anyway and rolled her eyes. “It’s not good to walk too close to the Schwarze Block, they can get violent.”

Indeed, the open-air festival vibe quickly died when police stopped the protesters, demanding that those who were hiding their faces remove their masks or leave. Individual demonstrators responded by throwing bottles and stones. Then the police broke into the crowd to try to separate the masked protesters by force.

One middle-age man threw himself to the ground and yelled “Fuck it!” and then, “I hate Germany.” A first-aid helper jogged up next to him and offered him a bottle of water.

The “Welcome to Hell” march had been organized through the Rote Flora, an old theater that was revamped as a squat for anarchists and the innovative left in the late ’80s. In recent years, older supporters of the Rote Flora have worried that the famous collective had turned into an autonomous bunker where members were not allowed to give interviews to the public without checking their answers with everyone else in the house first. But this week, the theater has won back some of its allure. Wednesday night saw a full house with cheering and music and fireworks being set off late into the night.

Meanwhile shops in the area around the Rote Flora, perhaps already anticipating a Strassenschlaft (street battle) between the hardcore left and the police, just like in the old days, have put up signs reading “Please don’t smash me” or battened down their windows with wood, on which notes were stuck that read, “We love you.“

On Friday, the summit begins, and the mood inside the conference hall may be only slightly more civil than it has been outside.

This article was updated at 2 a.m. ET.