The French government has threatened to ban all demonstrations after hard-left protesters smashed plate-glass windows of an operating theatre at a renowned Paris hospital for children.

The attack – while surgery was in progress – was a new low in the confrontation between the government, militant unions and the hard left over reforms in French labour law.

The health minister, Marisol Touraine, said: “There were children going into the operating room at the time. Some were asleep. This is truly shocking.”

The attack on the Necker children’s hospital occurred on Tuesday during the latest of a series of union-led demonstrations against plans to make French employment laws more flexible and business-friendly.

A group of hooded and masked left-wing or anarchist protesters made a short detour from the march to throw concrete slabs and paving stones through 15 windows of one of the most respected children’s hospitals in the world. Before they left the scene, one of the demonstrators wrote on a smashed window in red paint: “Ne travaillez jamais!" (never work). Groups of youths later pelted police with stones, injuring more than 20 officers, They also burned cars and smashed shop and bank windows.

Paris police chief Michel Cadot told the Associated Press that the incidents involved “extreme violence” not seen by officers in years. He said on Wednesday that 41 people were being held for questioning.

French President François Hollande, left, and Prime Minister Manuel Valls, right, both condemned the violence. (Jacky Naegelen/Reuters)

President François Hollande warned on Wedsnesday that the government might ban further demonstrations. “At a time when France is hosting the Euro 2016 [football championship] and faces the threat of terrorism, we cannot give permission for more demonstrations unless the safety of people and public property can be guaranteed,” he told the weekly cabinet meeting.

Prime Minister Manuel Valls said the hospital attack was “intolerable”. He accused the CGT of adopting a deliberately “ambiguous attitude” to violent protesters who attach themselves to union marches. Mr Valls suggested that union leaders welcomed the headlines that the violence brought while condemning the actions of a “minority”.

“I call on the CGT not to organise this type of demonstration in Paris any more,” Mr Valls said. Two further street protests are planned for later this month.

The Socialist-led government hopes that nationwide anger at the hospital attack will deflate what is already a rapidly declining protest movement. Attendance at marches across the country was high but well below the records promised by the militant union federation the CGT, which has spearheaded the protests over the past three months. Although rail and refuse-collection strikes are still in progress, their impact is weakening slowly.

Union officials accused the government of “panicking” and “trampling democratic freedoms”.