Hollande: France thwarted terrorist attacks this week

French President Francois Hollande has revealed that police thwarted several "terrorist attacks" this week.

Speaking on a trip to Marseille on Wednesday, Hollande said: "This week, we stopped terrorist attacks that could have taken place."

A security source later told the AFP news agency that four people with an "Islamist profile" had recently been arrested on suspicion of planning to attack military sites.

Speaking after Hollande, Interior Minister Bernard Cazeneuve said the four - aged 16 to 23 - were detained on Monday and that their leader had been in contact with known French jihadists in prison.

France has been on high alert since the Charlie Hebdo attacks in January and several subsequent suspected terrorist incidents

Bastille Day blasts

Hollande's comments came a day after two explosions at a petrochemical plant in Berre-l'Etang, near Marseille, which sparked huge fires.

No one was hurt in the incident which French officials have described as a "criminal act."

"We are treating this as arson," Aix-en-Provence prosecutor Dominique Moyal told reporters.

The explosions happened in the early hours of Bastille Day, or France's National Day, at a plant owned by the company LyondellBasell Industries.

Cazeneuve said investigators have been unable to uncover a clear motive for the blasts, which saw two tanks full of petrol and naphtha - a flammable liquid distilled from petroleum - catch fire. Cazeneuve had previously revealed that France had foiled several jihadist attacks over the past six months.

Debris from an electronic device had been found at the site, French media reported on Wednesday, quoting a source close to the investigation.

"Investigators have found elements that could constitute, if they are identified as such, a pyrotechnic chain," the source said.

Prosecutors said an explosive device was found in a third tank, but the device had failed to trigger a major explosion.

France remains vigilant

Security has been stepped up at key installations and public buildings since the attacks in January at the offices of Charlie Hebdo magazine and a Jewish supermarket in Paris, which left 17 people dead.

Last month, a man suspected of having links to the "Islamic State" militant group beheaded his boss and tried to blow up a US-owned gas factory near Lyon. He'd previously been on a terror watch list but was later removed.

More than 10,000 French troops are involved in anti-terrorism operations, and the military is actively recruiting additional numbers.

For the first time, members of France's anti-terror squad marched down Champs-Elysées in Paris for the Bastille Day parade on Tuesday.

mm/jil (AFP, AP, Reuters)