“I have full understanding for peaceful protest, but violent demonstrations are a threat to human life,” she said on Friday, after the first night of violence. “It is not acceptable.”
Hours later, 1,500 black-clad anarchists rampaged through the streets of the city’s Schanzen district, plundering shops and setting fire to cars and trash cans for several hours, the police said. An elite unit of special forces was called in to quell the violence, but only after extensive damage had been done.
The police said on Saturday that they had arrested 43 people in connection with the violence on Friday night. Ninety-six others remained in detention, pending an investigation. The police also said that 213 officers had been injured. Activists reported multiple injuries among the protesters, at least two of whom were hospitalized.
Hamburg has a long history of leftist politics, with occasional clashes between anarchists and the police. But the violence on Friday night rose to a level that shocked some longtime residents.
“What happened last night has nothing to do with legitimate criticism,” said Massimo Gugliotta, who was surveying a charred pile of rubble in the Schanzen district early Saturday. “Whoever did this was just using the idea of protest as an excuse for violence.”
Major streets in the city were blocked off to allow delegations of leaders to go to and from meetings. But many Hamburg residents, whose lives had been disrupted by the events, said they hoped to show that legitimate protests could be peaceful, and effective.
One contingent of about 12,000 people, galvanized by the theme “Solidarity Instead of G20,” began gathering early Saturday, the police said. Jan van Aken, a lawmaker with the Left Party, organized the demonstration with several other groups, including the Hamburg Refugee Council, local environmentalists and the Communist Party.
“Join the masses to send a signal against the G20, against the escalation and against meaningless violence,” Mr. van Aken said in a Twitter message. “Come out, now more than ever!”
In another part of the city, church and community leaders joined local politicians from Hamburg’s centrist parties to organize a demonstration that proclaimed “Hamburg Shows Attitude.” Several hundred people marched with blue, pink and yellow balloons, while others held signs with messages like “Make Global Trade Fair” and “Stop the Violence.”
Among the protesters was Michael Schmidt, 80, a writer, who had traveled to Hamburg from Munich with his son.
“We are fed up with the system” perpetuated by the Group of 20, Mr. Schmidt said. He denounced “the unquestioning of the capitalist system, the social irresponsibility and ecological irresponsibility” of the member nations. The Group of 20 comprises 19 industrial and emerging-market countries, plus the European Union.
Residents of the Schanzen district awoke Saturday to the smell of fire and the sight of shattered storefronts. The police said that about 500 protesters had looted a supermarket and then set it on fire as they retreated.
Videos posted on social media sites showed the smashed glass of a looted electronics store that sold Apple iPhones and other devices. Another showed masked demonstrators in black clothes plundering a grocery store; in yet another, masked protesters could be seen being chased by masked and heavily armed police officers against a backdrop of flames and smoke.
The authorities said the protesters were armed with homemade incendiary devices and iron bars. Activists said the authorities had turned water cannons against them more than 20 times on Friday.
As the evening began in the city’s St. Pauli section, about 30 protesters blocked roadways near the main train station with cinder blocks and pieces of a fence.
Dozens of police vans had to turn around because of the blockade, until about 200 riot police and two trucks arrived. The trucks used water cannons to disperse the protesters, who threw rocks at the vehicles.
Vanessa Lamm, a Schanzen resident who observed the protests in the district until about 2 a.m., said the police had waited to act until demonstrators began throwing stones and destroying property.
“Everybody went totally mad after that — the police and the people,” Ms. Lamm said. “When they started to destroy the shops, the police went in along with the special forces, who were stronger.”
By morning, residents were cleaning up trash and assessing the situation. Piles of charred bicycles and a refrigerator lay on the sidewalk. Dozens of shop windows — even those that had been taped with “No G20” and other slogans — had been reduced to shards.
Kristin Litzbarski, an employee at the Oma’s Apotheke pub, found that her workplace had been spared.
“We were lucky, but so many other stores, owned by people I know right next to us, were not,” she said. “It looks like a war zone. I’m really angry at people for destroying our home. It’s O.K. when people demonstrate, but where does all this aggression come from?”