Hundreds of protesters attempted to forcibly enter areas cordoned-off by authorities, known as the "high security zone", to upset the schedule for the first day of meetings between international leaders.

Cars have been set ablaze at various locations throughout the city while protesters blocked an important junction at the city's harbor.

Around 19,000 police officers have been deployed to ensure the safety of the global dignitaries attending the summit. Several police helicopters are circling the skies above Hamburg and a ban on public gatherings is in place in parts of the city until the summit ends at 5:00 p.m. local time (1500 GMT) on Saturday.

Federal police have reported an arson attack on one of its stations in the Altona district where protesters threw firebombs, causing damage to three police vehicles.

German special police forces walk through the Schanze district following clashes with anti G20-protesters in Hamburg, Germany, July 8, 2017. [Photo/Xinhua]

According to police information, several junctions in the inner-city are being blocked. Police surrounded around 200 protesters from the group "Block G20 - Color the red zone" which seeks to block access roads to the summit and infiltrate the high security zone. The group has warned authorities of "mass, announced, public disobedience."

Police used water cannons at a sit-in protest on one of the streets designated for U.S. President Donald Trump's journey to the summit.

Meanwhile, more than 1,000 individuals clad in white and purple gathered at the city's piers and shouted "Get out, get out."

Eyewitnesses said police attacked participants in the demonstration with clubs.

Clashes between 200 protesters and police were also reported in the Berliner Tor area. A member of police described the situation as "very dynamic."

Protests caused traffic congestion as trucks were unable to access the harbor. Earlier on Friday morning, there had been a temporary blockade of railway tracks leading to numerous delays until police removed the protesters from the tracks.

Friday's demonstrations marked the second day of escalating clashes between protesters and German authorities. On Thursday night, a demonstration dubbed "Welcome to Hell" caused widespread material damage in the Altona district and left 111 police officers and several protesters injured.

A total of 29 individuals were arrested, a member of police said on Friday. While she could not say exactly how many protesters were injured, the number was likely to be high and some had suffered serious injuries.

The police union GdP (Gewerkschaft der Polizei) defended the use of force by authorities at the "Welcome to Hell" demonstration. According to GdP director Oliver Malchow who spoke to German radio station Deutschlandfunk on Friday, security forces had to ensure the right to peaceful assembly and prevent individuals from committing crimes.

Also speaking to Deutschlandfunk, member of police Timo Zell said the situation had threatened to spin out of control and cited 3,500 people at the demonstration who announced their intent to use violence to get their message across.

According to Zill, police negotiated with masked protesters for 45 minutes before coming under attack from people throwing bottles, iron bars and roof tiles. Zill said he had "never witnessed anything like it" in his career, and described the subsequent police intervention as being "without alternative."