Leaders of the world's biggest economies met in Hamburg on Friday to discuss issues ranging from climate change to trade.
Protests against globalisation and capitalism have been held, while all eyes were on the first face to face meeting between Presidents Trump and Putin.
TRT World's Simon McGregor-Wood reports from Hamburg.
Protesters clashed with police, torched patrol cars and blocked roads in the German city of Hamburg on Friday in fresh violence just before the start of the G20 summit, police said.
"An operation is under way against violent individuals" who threw petrol bombs and set fire to patrol cars near a police station in the city's Altona district, federal police said. At least 160 police officers were reported injured in the clashes.
A German officer fired a warning shot in Hamburg on Friday evening as the protests raged on for a second straight evening, police said.
"A police officer was attacked by several violent people.. and he fired a warning shot," police said on Twitter. The officer then fled into a shop and was rescued.
In a subsequent tweet, police clarified that the incident was not linked to the demonstrations against the G20 summit, but was connected to a street robbery.
"The warning shot was discharged when police observed a street robbery, intervened and were attacked by the assailants," it said.
Police forces around Germany dispatched reinforcements to help 15,000 police already deployed to the northern port city for the G20 summit as the violence escalated. At least 15 people were arrested and dozens more held for questioning.
TRT World spoke with journalist Matt Larotonda about the latest round of clashes.
It was unclear how many protestors were hurt. Organiser Andreas Blechschmidt criticised what he said was a heavy-handed and "massive" police response with batons.
"The police should have reacted proportionally ... It wasn't necessary. There are a lot of people injured," Blechschmidt said on N-TV. Around 30 people were arrested.
"War, climate change, exploitation are the result of the capitalist system that the G20 stands for and which 20,000 police are here to defend," demonstrator Georg Ismail said.
In the west of the city, a "plume of black smoke" was rising, and cars in some areas had been set alight, the local Hamburg police said separately.
In another incident, protesters attempted to shoot a police helicopter with a flare, which police confirmed. Twitter user Anna-Lena Punken captured the moment and tweeted it.
There was no damage caused to the helicopter.
Police said demonstrators had blocked several intersections and so-called transfer corridors – roads designated to help delegations move between meetings.
"Welcome to hell"
Dozens of police were wounded in other clashes on Thursday, during which protesters destroyed cars, set fire to flames, threw bottles and raised a slogan telling US President Donald Trump and other leaders of the summit, "Welcome to hell."
At this two-day summit, leaders of the world's largest economies are meeting to resolve deep differences with Trump over climate change and trade.
The Hamburg meeting comes at a time when the global political scene is undergoing radical transformations under Trump's policies, which introduced the motto of "America First" and pushed Europe and China to consolidate their relations.
The protests have not been appreciated by many locals as the violence erupted.
Local resident Benjamin Laub, 53, said his neighbourhood has resembled a zone "under a state of emergency" for days.
"For a week we've heard the choppers above all the time. The buses don't run, people leave their cars at home, they walk or ride bicycles. I'm carrying my passport for the first time in Germany (because of police checks)," said the hockey trainer.
A livid Markus Munch, 48, speaking on his bike near Trump's residence in a plush area beside the Alster lake twinkling in the summer sunshine, had little sympathy for the "idiots" demonstrating.
"It's a total catastrophe that a minority can just riot like this," Munch tsaid. "These idiots have nothing better to do than to demonstrate. They have to be moved out."
The setting of cars alight by protesters was captured on film and shared on Twitter.