German police used tear gas and water cannon Thursday as a stand-off between authorities and thousands of protestors against a multi-billion-euro (dollar) rail project escalated.

Numerous protestors were injured, said Gerhard Pfeifer, a spokesman for a group organising regular mass demonstrations against the "Stuttgart 21" project that is turning into a major political hot potato for Chancellor Angela Merkel.

Between 5,000 and 10,000 protestors tried to prevent construction workers from cutting down 25 trees in a park near the central station in Stuttgart, southwestern Germany, Pfeifer said.

Police said they had deployed 1,000 officers to disperse the protestors. They said around 1,000 people took part in the protest.

The building work is part of a seven-billion-euro (9.5-billion-dollar) plan that aims to make Stuttgart and the surrounding region part of the 1,500-kilometre (930-mile), high-speed "Magistrale for Europe" across Europe.

Opponents say the project will take longer than expected, go massively over budget and ultimately do little to speed up rail traffic. They say that other parts of Germany's rail network are in bigger need of improvement.

But locals in Stuttgart most object to parts of their beloved train station, built between the wars by architect Paul Bonatz, falling victim to the wrecking ball, and to trees being cut down.

Last week, Merkel surprisingly threw her support behind the project.

It looks set to become a major issue in a March vote in Baden-Wuerttemberg state, where the chancellor's conservatives could lose control after more than half a century in power.