Seven suspects were arrested in police raids last weekend in the eastern city of Strasbourg and Marseille in the south following an eight-month investigation by security services, although two were later released.
The source said investigators found the suspects had made internet searches on sites including the Christmas market on the prestigious Champs-Elysees avenue, the Disneyland Paris theme park, cafe terraces in the northeast of the capital, the Paris criminal police headquarters and a Metro station.
Five of the suspects have had their custody extended under legislation covering investigations into an imminent terror attack.
One of the men is said to have told investigators that attacks were planned and named the police headquarters in central Paris and the DGSI domestic security department in the northwest of the city as targets.
Some members of the group are thought to have taken orders from a Syria-based jihadist.
Two handguns, an automatic pistol, a submachine gun and jihadist propaganda were all found during the raids in which they were arrested.
France is under a state of emergency that gives security forces enhanced powers of surveillance and arrest, a year after the Paris attacks.
Islamist extremists have carried out three large-scale attacks in France since January 2015, when gunmen targeted the Charlie Hebdo satirical magazine and a Jewish supermarket.
Ten months later, Islamic State jihadists massacred 130 people in attacks on the Bataclan concert hall, France's national stadium and a handful of bars and restaurants in eastern Paris.
And in July, a self-radicalised extremist ploughed a truck into crowds watching fireworks in the southern city of Nice, killing 86.