French intelligence services are searching for a female would-be suicide bomber who they believe is planning to attack the Paris transport system.
The country is on full alert against a specific terrorist threat that has been confirmed by two separate sources, according to French radio.
The rector of the Grand Mosque of Paris, Dalil Boubakeur, considered a moderate Muslim, has also been given three armed guards after a spokesman said he was under a "real threat".
France's interior minister, Brice Hortefeux, refused to give details of the threat but said "our vigilance is reinforced today".
The anti-terrorist court in Paris has opened a preliminary inquiry to "establish the credibility" of the reports of an imminent terrorist attack. According to a ministry of interior spokesman, the alleged woman bomber was "not necessarily the most worrying" intelligence information. He did not give further details.
"All the lights are flashing red," said another intelligence source.
Last week, Hortefeux admitted the threat was at a "high level" when he visited the Eiffel tower after it was evacuated following a bomb alert. The Saint-Michel metro station near Notre Dame cathedral was also briefly evacuated over a similar threat.
France's terrorism level has been on what is described as "reinforced red" since 5am last Thursday. This followed intelligence from a North African country, said to be either Morocco or Algeria, that an Algerian was planning to carry out an attack on a Paris train, bus or metro that day. French intelligence had been given similar information from another source.