The incident – captured in video footage posted on social media this week – occurred as riot police clashed with violent protesters during the march on Wednesday.
The officer, surrounded by fellow riot police, appears on the footage to be throwing the stone several meters towards the demonstrators.
- Why did dozens of protesters burst into a Paris hospital during May 1st demonstrations?
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— Le Parisien (@le_Parisien) 3 May 2019
The general inspectors' office of the national police (IGPN) has now opened an investigation at the request of French police chief Didier Lallement, police sources told AFP.
Prosecutors have also opened a judicial investigation, legal sources said.
“If someone is at fault, there will be a sanction, legal and administrative sanctions,” Interior Minister Christophe Castaner told journalists during a visit to Toulon in southern France.
The traditional May Day rallies for workers' rights were joined this year by “yellow vest” protesters who have staged weekly Saturday protests since November to demand tax cuts and improved living standards.
Some protesters set fire to bins, furniture and several cars. Photo: AFP
But those protests have often degenerated into rioting and violent clashes with police, in particular by anti-capitalist “black bloc” youths, dozens of whom also took part in Wednesday's protest.
For months, yellow vests have accused the police of heavy-handed repression of their right to assemble and protest, in particular the use of rubber bullet launchers that have seriously injured dozens.
Tensions ran high throughout Wednesday afternoon, with dozens of protesters at one point pouring on to the grounds of a hospital and later trying to enter an intensive-care unit.
Officials quickly condemned what Castaner initially described as an “attack” on the hospital, although cellphone footage by nurses released on Thursday suggested they were only seeking refuge.
Castaner acknowledged on Friday that “I shouldn't have used the word 'attack',” saying the incident was more of a “violent intrusion.”
“I have no problem recognising that I misspoke,” he said.