Riot police have fired water cannon and tear gas at protesters who hurled bottles and bricks on the eve of the G20 summit in Germany.

Officers in helmets and equipped with riot shields faced off against the demonstrators on the streets of Hamburg.

US President Donald Trump and other world leaders are due in the city tomorrow for the G20 summit.

The demonstration was dubbed 'Welcome to Hell' by anti-capitalist groups.

Police said the trouble involved around 1,000 people, but as many as 100,000 are expected to descend on the city to take part in protests during the two-day summit.

Reporter Alex Rossi, on the scene, said the violence - near the city's fish market - was expected and that police had been preparing for trouble as demonstrators attempted to march through the city.

He said a few had been carried off with what appeared to be "quite serious" injuries.

"When we arrived earlier the atmosphere was peaceful and calm, people drinking and enjoying the sun, listening to speeches," she said.

"Within that group there were people who looked like they had come here to cause trouble.

"As it got to early evening we saw a number of people coming down dressed in black, some of them masked with handkerchiefs across their faces."

The trouble appeared to be dying down after several hours and the situation is improving.

The G20 countries - whose members represent about 80% of the world's population - are meeting with their usual broad remit of making the world a more stable and prosperous place.

Climate change is set to be a major talking point after President Trump withdrew the US from the landmark Paris Agreement.

All eyes will also be on his meeting with Russia's Vladimir Putin, although it is not certain if the pair's first exchange - and handshake - will be caught on camera.