Paris' prosecutor has begun a probe after May Day protesters allegedly entered a famed Paris hospital and tried to force the door to its intensive care unit.

But supporters of the "gilets jaunes" ("yellow vests"), who were among demonstrators during a fiery march in the French capital on Wednesday, said they were just seeking refuge from tear gas fired by police.

Thirty-two people were in police custody under charges of "gathering to commit degradations or violence," the prosecutor's office told Euronews.

They were released on Thursday evening as investigations continue, the prosecutor's office said.

France's Interior Minister Christophe Castaner initially said the hospital was 'attacked' by dozens of anti-capitalist militants and black blocks, but by Friday morning he had changed his position.

"I shouldn't have used the word 'attack'. The term 'violent intrusion' used by the hospital director seems to be more accurate giving the videos on it shared ever since," admitted Castaner during a press conference.

Hospital director Marie-Anne Ruder said the gate had been forced open and she saw dozens of people going inside, some wearing the protesters' trademark high-visibility yellow vests and others with their faces covered.

She called the police because of their "violent and threatening behaviour", she told France Inter radio.

"Several dozen people tried to force the door into the intensive care unit," she told RTL in a separate interview, saying the security forces had turned up 10 minutes later to remove the intruders.

Paris hospitals director general Martin Hirsch said CCTV footage showed a number of intruders trying to get in as a group of nurses struggled to hold the door shut, shouting: "Be careful, there are patients in here!"

At the time of the incident, a riot police officer was being treated there for a head injury, but Hirsch said it was not clear what motivated the "inexplicable intrusion".

It drew sharp condemnation from the government, with Prime Minister Edouard Philippe denouncing it as "totally irresponsible".

French Health Minister Agnes Buzyn described it as "unspeakable".

"This is the first time there has been an act of violence at a hospital," she told Europe 1 radio. "I think that all French people, like me, are extremely shocked."

'Seeking refuge from tear gas'

But supporters of the "yellow vest" movement, whose protests have shaken the government of President Emmanuel Macron over the past half year, insisted the demonstrators were merely seeking refuge from tear gas fired by police.

The incident came during a hugely tense May Day which saw police clash with hardline protesters on the sidelines of the annual labour union march. The hospital is close to the Place d'Italie where the march ended.

Salome Fournet-Fayas, a 26-year-old set designer who was demonstrating on May Day, was at the hospital with other protesters.

She told Euronews that she and other demonstrators were following the official itinerary for the rally when police fired tear gas at them in such quantity that she almost vomited.

Fournet-Fayas said that as demonstrators were fleeing tear gas and possible flash ball shots, they noticed that the hospital gates were open. So they entered and quietly waited for things to calm down on the grass, in the hospital courtyard, without going inside the building.

Asked whether some violent protesters might have tried to attack the hospital, she replied that "everyone was very calm."

She said she felt "revolted" by the government's account of the events. "No one tried to attack to attack this hospital," she assured. "We were just there waiting."

READ MORE: May Day protests turn violent in Paris

While acknowledging some people may have entered the hospital to seek shelter, Buzyn suggested others might have been bent on theft, notably of hi-tech equipment.

The situation would be clearer after those in detention had been questioned, she said.

Jean-Luc Melenchon, leader of far-left France Unbowed party, accused Castaner of making up what he described as a "pseudo attack" on the hospital.

"Truth is the first victim of Macron's sidekicks," he wrote on Twitter.