Officers dressed in riot gear intervened as protest groups tried to enter the red zone — the blocked-off area close to the summit venue — while other small groups staged sit-ins across the city.
Police said 159 officers have been injured since Thursday and reinforcements from outside Hamburg have been requested.
At one of the sit-ins, a little over a mile from the summit security zone, water cannon were deployed against protesters who had come armed with umbrellas and rain gear. Earlier, protest organizers told CNN they would try to storm police barricades around restricted areas.
Members of the “Color The Red Zone” protest said they were trying to make it difficult for G20 participants to travel to the talks.
“We are living in a democracy and the red zone is not a democracy,” said Karl S, a student protester from Düsseldorf who declined to give CNN his full name.
“I’m sure we can’t stop all these leaders from meeting but if we can stop them from getting their food or catering shortly we’ve achieved something,” he said.
Christian, one of the protest organizers who also declined to give his surname, said the “point is to disturb the G20,” adding: “It’s not right that a few countries get to decide what happens to the rest of the world at this summit.”
Video from nearby showed police dressed in riot gear aggressively pushing a group of protesters and journalists away from the scene.
“This is what democracy looks like,” protesters shouted amid the chaos.
Melania Trump’s schedule impacted
Some small groups of people were pushed back as they tried to march down streets to be used by G20 delegates. They said their objective was not to get close to the summit but to block the delegates’ routes to it.
Volunteers could be seen treating people with saline solution, possibly due to tear gas or pepper spray, and tending to other wounded. One woman could be seen bleeding from her neck or head. Police told demonstrators they were in an area that was forbidden.
Protesters appeared to disrupt some G20 visitors’ movements around the city.
First Lady Melania Trump’s spokeswoman told reporters she was unable to take part in scheduled events with other G20 spouses due to the protests. “Hamburg police couldn’t provide clearance for us to leave,” Stephanie Grisham said.
The spouses were due to take a river cruise and tour a climate change center, according to media reports.
Hamburg police tweeted Friday afternoon that some officers had been attacked with iron bars on the Reeperbahn, the city’s famous entertainment district.
Other officers were attacked with fire bombs in Holstenstrasse, further north, police said, and barricades were set alight.
Violence also flared in the city’s Altona district Friday morning, a couple of miles to the west of the summit security zone.
Hamburg police said one of their helicopters was targeted with a flare which only narrowly missed it. Two police vehicles carrying officers were also attacked, with paint thrown and windows smashed. Bottles and fireworks were flung, injuring some officers, police said.
In addition, the tires of a Canadian delegation car were slashed, according to police.
Police earlier asked the public to avoid the area near the Hamburg-Altona Railway Station, where authorities said people had set vehicles aflame and thrown Molotov cocktails. Smoke could be seen across the skyline from fires said to have been lit by protesters.
The latest round of demonstrations, though unruly, are smaller and more dispersed than Thursday night’s violence, during which at least 111 police officers were hurt, according to police. Water cannon were deployed to disperse protesters and 29 arrests were made. Police did not give the reasons for the arrests.
Security is tight in the city of 1.8 million, as leaders from around the world convene for the G20 summit.
The Group of 20, which includes 19 countries and the European Union, accounts for approximately 80% of global GDP. Around two-thirds of the world’s population live in a G20 country.
During the two-day meeting in German Chancellor Angela Merkel’s birthplace, leaders are expected to discuss climate change, terrorism and migration.
While many of the demonstrators who gathered on Thursday were protesting against capitalism, some joined to voice their concern about issues including global warming.
CNN’s Kara Fox reported from Hamburg, while Josh Berlinger wrote from Hong Kong and Laura Smith-Spark from London. CNN’s Atika Shubert and Kevin Liptak contributed to this report.