German environmentalists vowed on Saturday to keep up protests against a Stuttgart railway project, emboldened by criticism of the government after clashes between police and demonstrators.

"We will organise more huge protests," said Gangolf Stocker, a spokesman for a campaign against the redevelopment of Stuttgart's railway station, which has been defended by Chancellor Angela Merkel and her political allies.

On Saturday, dozens of environmentalists camped in tents and improvised treehouses in a park outside the station where workers have begun chopping down trees as part of the station expansion.

On Thursday, more than 100 people were injured when police clashed with protestors. The images of the violence shocked Germany, where such confrontations are rare.

Between 50,000-100,000 people attended a second demonstration on Friday evening, which passed off peacefully.

The next demonstration is due to take place on Monday evening and is expected to draw a crowd of 20,000 people, said Berthold Friess, a leader of the green BUND movement.

The government's handling of the protests was criticised by German media on Saturday.

Berliner Zeitung said the situation was a "disaster" for Merkel's Christian Democratic Union, as she risked "endangering the legitimate right to defend conservative values such as law and order".

"The state cannot win the argument with police, water cannon, pepper spray and chainsaws," said Sueddeutsche Zeitung.

The demonstrators have been protesting a seven-billion-euro (9.5-billion-dollar) project that aims to make Stuttgart and the surrounding region part of a 1,500-kilometre (930-mile), high-speed rail route across Europe.

Residents of the wealthy city object most of all to parts of their train station, built between the wars by architect Paul Bonatz, being demolished, and to hundreds of trees, many of them old, being cut down.