Demonstrations turned violent ahead of a tricky first G20 summit for US President Donald Trump, as German police clashed with a hard core of masked anti- capitalist activists hurling bottles and stones.

What should have been a peaceful march on Wednesday by around 12,000 people in Hamburg protesting against globalisation was halted as police used water cannon and tear gas to disperse around 1,000 far-left militants.

Nine people were injured including seven police officers and two demonstrators, a police spokeswoman told AFP. There were also several arrests.

Police called with loudspeakers on protestors to remove their masks but this was ignored and after more objects were thrown, authorities decided to separate them from the other protestors, police said on Twitter.

"Unfortunately it has come to the first clashes. We are implementing corresponding measures," read another tweet.

Protesters were seen scrambling to leave the scene, while others defiantly stood in the way of water cannon trucks as they moved in surrounded by riot police with helmets and batons.

Police tweeted a photo of a car and flames and said shop windows were smashed.

The main "Welcome to Hell" march was then called off but thousands of people remained as night fell intending to march and riot police and demonstrators engaged in smaller skirmishes in the side streets of Germany's second city, AFP correspondents at the scene said.

Up to 100,000 demonstrators are expected before and during the two-day Group of 20 meeting gathering Trump, Russia's Vladimir Putin and Xi Jinping of China starting today.

There were 20,000 police on standby together with armoured vehicles, helicopters and surveillance drones. A holding centre for detainees has been set up in a former hardware store with space for 400 people.

Ugly scenes had already unfolded Tuesday night as riot police used water cannon and pepper spray to clear an unauthorised protest camp, leaving five people injured and driving fears of more trouble ahead.

Major events like the G20 have in recent years usually been held in remote locations, but Germany was forced by its logistical demands to host it in a large city with a big venue and dozens of hotels.

Hamburg is desperate to avoid a rerun of the kind of major clashes seen at the 2001 G8 summit in Genoa or the Frankfurt opening of the new European Central Bank building in 2015.

In Hamburg, some 30 demonstrations have been announced, organised by anti-globalisation activists and environmentalists, trade unions, students and church groups.

"Welcome to Hell" organiser Andreas Blechschmidt said the motto is "a combative message… but it's also meant to symbolise that G20 policies worldwide are responsible for hellish conditions like hunger, war and the climate disaster".