French police repeatedly used tear gas and water cannons to break up a protest Saturday by nearly a 1,000 yellow vest demonstrators in the southwest city of Toulouse.
A police statement said officers made five arrests after being targeted by missiles thrown by some of the protesters.
A group that observes police conduct at yellow vest protests said officers had attacked five of their number during the demonstration, injuring one of them.
The Observatory of Police Practice (OPP) posted images and video on Twitter to support their account and posted an open letter to the authorities protesting the incident.
The police headquarters in Toulouse was not available to comment on the allegations Saturday evening.
Earlier this month, a member of the OPP filed a complaint alleging that he had been injured during a police charge at a yellow vest protest.
The march in Toulouse, which holds regular yellow vests protests on Saturday, was led by demonstrators brandishing a giant banner that read: "Fed up of surviving. We want to live."
They also burnt a parasol in front of a McDonalds outlet.
In the capital Paris, some yellow vests joined a climate protest march.
Saturday's protests came two days after the French government unveiled a draft 2020 budget with more than nine billion euros in tax cuts for households.
It includes five billion euros in tax cuts for some 12 million households already promised by President Emmanuel Macron, the result of a "great national debate" he held to try to address the ongoing protests.
Macron swept to the presidency in 2017 with a pledge to get the country back on a solid financial footing. But he was caught short by the "yellow vest" movement which accused the former investment banker of ignoring the day-to-day struggles of many French.
Demonstrations have been banned on the Champs-Elysees after protesters clashed with police on the famous Paris avenue on December 8 last year, in the early days of the yellow vest protests.
On that weekend, police detained 900 people: the most since the anti-government protests began.
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