About 80 riot police rushed to the building early this morning to remove at least 50 students who barricaded themselves inside a classroom.

Police pushed through and the students moved to an inner courtyard under the dome of the university. Under the threat of continued police aggression and mass arrests they choose to leave with three arrests.

The sit-in at the university took place as part of growing national protests against a youth employment plan that will make it easier for firms to fire workers aged younger than 26.

The government hopes the flexibility will encourage employers to hire young people, safe in the knowledge that they will be able to get rid of them if necessary. It mean that younger workers get less job security than older colleagues and undermine France's generous employment protections.

"It's about our future, and we are determined not to give up," said Elisa Penisson, a 21-year-old undergraduate at the Sorbonne.

A university administrator, Nicolas Boudot, said the protesters wanted to turn the university into "a battlefield", not only against the jobs measure "but also against all of the social problems" that France is facing.

This mornings police action was apparently the result of demands from the rector of the Paris Academy. On Friday, the cops had fire chemical weapons into the building claiming that they were coming under attack from students throwing books from windows.

In Tours, 200km southwest of Paris, several hundred students moved onto tracks at the railway station, stopping trains for three hours on Friday, the SNCF rail operator said. Students picketed entrances at several of the country's more than 80 universities. The main students' union said 45 colleges were affected, though Education Minister Gilles de Robien dismissed those figures as "lies."

Interior Minister Nicolas Sarkozy decided to return early from an all expenses visit to the French West Indies because of the demonstrations.