French police suspect that al-Qaida affiliates in North Africa may be plotting a major bomb attack on a crowded target in France.
National police director Frederic Pechenard told French radio on Wednesday that threat levels had hit a "peak."
He said police had "serious indications" of an attack, and that al-Qaida in North Africa was targeting France in particular.
Pechenard said authorities are focusing on possible assassination attempts, as well as potential mass casualty attacks in crowded urban areas.
Al-Qaida in the Islamic Maghreb claimed responsibility for kidnapping five French workers in Niger last week.
The threats follow last week's parliamentary vote to ban the public use of the Muslim Burqa-like veil.
On Monday, France Interior Minister Brice Hortefeux said the government had stepped up security measures, believing that the threat of a terror attack in France was real.
The veil ban, backed by President Nicolas Sarkozy and approved last week by the French senate, makes it illegal throughout France for Muslim women to wear the full-face veil, which leaves only the eyes uncovered.
French authorities said reports that a female suicide bomber might have been planning a suicide attack in Paris were no longer credible.
Some information for this report was provided by AP, AFP and Reuters.