The Council of Europe -- the continent's leading human rights watchdog -- on Friday criticised French police's "excessive use of force" during demonstrations against an unpopular pension reform.

"Violent incidents have occurred, including some that have targeted the forces of law and order," its commissioner for human rights Dunja Mijatovic said.

"But the sporadic acts of violence of some protesters or other reprehensible acts committed by other persons during a protest cannot justify excessive use of force by agents of the state. These acts are also not enough to deprive peaceful protesters of their right to freedom of assembly," she said.

"It is up to the authorities to allow the actual exercise of these freedoms by protecting peaceful demonstrators and journalists covering these protests against police brutality and against violent individuals acting within or on the sidelines of marches," she added.

French President Emmanuel Macron faced mounting pressure on Friday after violent demonstrations that left more than 400 security forces injured and the centre of major cities shrouded in tear gas and smoke.

More than 450 people were arrested Thursday during the most violent day of protests since the start of the year against Macron's bid to raise the retirement age to 64.

The French presidency announced Friday that King Charles III's visit next week had been postponed after unions declared another day of strikes and protests on Tuesday, during what would have been the British monarch's trip.

Rights groups, magistrates, and left-wing politicians have in recent days raised the alarm over what they have described as arbitrary arrests and apparent abusive police practices during protests against President Emmanuel Macron's plan to raise the retirement age from 62 to 64.

Anger has boiled over in the street since his government last week invoked a controversial executive power to force through the bill without a parliamentary vote.

Police have detained hundreds as they try to disperse daily protests in Paris and other cities, but rights advocates say they have released the large majority without pressing charges.

Amnesty International has raised the alarm over "widespread use of excessive force and arbitrary arrests".

Reporters Without Borders has said police assaulted several "clearly identifiable" journalists.

But security officials have defended their actions, saying they are responding to violent rioters and anarchist groups which frequently infiltrate French demonstrations to provoke clashes.

More to follow.