PARIS — French authorities arrested five people accused of encouraging attacks on a hotel slated to accommodate police during this weekend's G7 summit in Biarritz, a source close to the inquiry said on Tuesday.

The arrests occurred on Monday morning, just days ahead of the summit chaired by French President Emmanuel Macron who will from Saturday host the likes of Donald Trump, Angela Merkel and Boris Johnson in the glitzy resort in southwestern France.

Several of the suspects are young members of radical anti-capitalist groups known as "black blocs" which played a major role in violent street protests that rocked Paris and other French cities over the last months.

The investigation was triggered after a woman living in the area posted a message online pinpointing a hotel where gendarmes policing the summit will stay.

Four people suspected of issuing calls to "burn down" the hotel were arrested in locations including Hendaye, south of Biarritz, and Loire Valley town of Tours. One of them has a police record for violence during a demonstration, the source said.

The woman who published the initial message, from the Landes region north of Biarritz, was also detained.

A raft of unprecedented security measures has been put in place ahead of the summit, with the resort on lockdown and its picturesque Grand Plage beach off-limits to everyone except delegates and those accredited for the summit.

Activists have announced a "counter-summit" and some 10,000 security forces have been drafted in to ensure there are no disruptions in a country that has been hit by months of mass anti-government protests.

The protests were spearheaded by so-called "yellow vests" activists protesting against social inequalities although more radical groups like the Black blocs also got involved.

Police will be under huge pressure in Biarritz to keep order without resorting to heavy-handed tactics that sometimes marked their response to the street protests.

Since Monday morning, anti-capitalist activists, environmentalists and other anti-globalization players have begun rallying some 20 km to the south in Urrugne near the Spanish border where they have planned a week of protests they insist will be peaceful.

A large anti-G7 rally will take place in Hendaye on Saturday to coincide with the formal opening of the summit. — AFP