A French gendarme officer patrols in front of the Arc de Triomphe, on the Champs Elysees, in Paris, Monday Nov. 23, 2015. AP Photo/Laurent Ciprian

Four arrested on suspicion of planning an "imminent" terror attack in Paris

Suspects reportedly caught building explosive devices favoured by ISIS

Arrests on Friday morning follow investigation on social media

Four people, including a 16-year-old girl, have been arrested in the south of France on suspicion of planning an attack in the French capital, the Paris prosecutor's office said.

The four people, who were monitored as part of a preliminary investigation, were arrested in Montpellier on Friday morning by anti-terror police.

They were allegedly planning an attack with a suicide belt in a tourist hotspot in the French capital, according to M6.

Other French media report police did not know where or when the attack was going to take place, only that it was "imminent."

A teenage girl, who was supposed to serve as "bait" during the attack and her 20-year-old boyfriend were arrested, both were reportedly known by authorities for their connections with radical Islam and desire to travel to Syria.

According to BFMTV, the couple planned to marry. He then planned to carry out the suicide attack, while she travelled Syria. The girl reportedly recorded a video of herself pledging allegiance to ISIS last Wednesday, French broadcaster LCI reports.

The other two suspects were 27 and 34 years old. The oldest one was supposedly their "tutor" and had contacts in Syria.

Police discovered 71 grams of TATP, a powerful explosive — used during the November 13 Paris attacks and March 22 Brussels attack — when they raided one of the suspect's flat. The explosive is reportedly favoured by ISIS, but there is no evidence yet that the terror group is behind this latest plot.

They also found a laboratory set up to make the substance as well as one litre of acetone, hydrogen peroxide, and one litre of sulfuric acid — the three ingredients needed to make the explosive, BFMTV reports.

The girl, her boyfriend and the oldest suspect were arrested as they tried to buy acetone, the fourth was arrested because he was at one of the suspects' homes. Police have not determined his involvement in the plot.

The arrest reportedly follows investigation on social media, which allowed the police to identify the four suspects. Two other police operations, in Clapiers and Marseillan took place on Friday morning in relation with the Montpellier arrests.

French police secure the site near the Louvre Pyramid in Paris, France, February 3, 2017 after a French soldier shot and wounded a man armed with a knife after he tried to enter the Louvre museum in central Paris carrying a suitcase, police sources said. REUTERS/Christian Hartmann which "made it possible to foil a plan for an imminent attack on French soil." An inquiry was opened by the anti-terrorism section of the Paris prosecutor's office two weeks ago, he added.

Prime Minister Bernard Cazeneuve also reacted to the arrest, saying that the very high terrorist attack threat to the nation meant there was a big mobilisation of intelligence services. He added that since the beginning of 2016 the police had brought in several hundred of people for questioning.

More than 230 people have died in France in a series of Islamist militant attacks since the beginning of 2015. The country has been under a state of emergency rules since the November 2015 attacks that killed 130 people and injured hundreds more.

The latest attack took place last week when a man screaming "Allahu Akbar" attacked soldiers with a machete at the Louvre museum in Paris.