Thousands of people have taken to the streets of Hamburg, Germany, to protest against capitalism, climate polices and globalisation among other issues as world leaders gather for the two-day G20 summit.

Demonstrators and officers clashed in some neighbourhoods on Friday, prompting the Hamburg police to request additional reinforcements to help cope with the scale of the protests.

According to officials, about 20,000 police officers were already on duty for the two-day summit where the leaders of the world's top economies will tackle everything from trade to climate change to major conflicts.

Some activists smashed windows and torched patrol cars and other vehicles on Friday morning, while riot police used water cannon and pepper spray on activists.

A police spokeswoman also said that at least 159 officers had been injured and at least 45 people had been arrested.

More than 100,000 people are expected to take part in demonstrations and marches across Hamburg on Friday and Saturday.

Those in the streets are protesting against a number of issues, including capitalism, climate polices and globalisation.

"There are so many issues involved politically in the G20 summit," said Al Jazeera's David Chater, reporting from Hamburg .

"They are against the globalist capitalist system, they are against Trump's attempts to pull out of the climate change accords. They're essentially protesting every single issue that's going to be raised in this G20 summit."

Trump-Putin talks

President Trump and Russian President Vladimir Putin had a brief first encounter on Friday when they shook hands and exchanged pleasantries.

Al Jazeera's James Bays, reporting from Hamburg, said Trump and Putin have "important issues to discuss, particularly Ukraine, but also the issue of Syria where it is believed that there may be a new initiative coming from Russia and the US to try to get the political track moving and to try to get some stabilisation zones around Syria".

Trump's meeting with Putin will take place at the same time as a closed-door meeting on climate change, which could be "a bit of a sign" about Trump's policies on the climate, Bays said.

Also on Friday, protesters blocked the route of US First Lady Melania Trump, according to the German DPA news agency.

A White House spokeswoman told Al Jazeera that the first lady's motorcade could not be given clearance to leave because of the security situation.

Many other groups are calling for peaceful protests and are pushing the G20 leaders for action to fight climate change and address economic disparities in the world.

Some are even calling for the dissolution of the G20 itself so that the United Nations becomes the platform for such discussions.

The G20 summit comes at a time of major shifts in the global geopolitical landscape, with US President Donald Trump's "America First" policies pushing Europe and China closer together.