France has raised security to the second-highest level as it prepares to host world leaders to mark the 60th anniversary of the D-Day landings.
Patrols have been beefed up around the country
Some 9,000 troops are being deployed in the northern region, and any planes
violating a no-fly zone in Normandy will be shot down, the military warned.
Surface-to-air missiles have already been installed along the coastline.
A host of world leaders, including US President George Bush, are to attend the weekend WWII anniversary event.
Among the guests will also be Chancellor Gerhard Schroeder - the first German leader to be present at such an event.
Some 132,500 Allied troops stormed the beaches of Normandy on 6 June 1944 to drive German forces out of France.
On Tuesday, the Vigipirate security plan was stepped up to the "red" level across the country, French interior ministry officials said.
The D-Day landings were a turning-point in World War II
The move allows random checks at railway stations and beefing-up of patrols across the country.
France had already resumed border controls - permitted under the Schengen agreement on free movement within the European Union when national security requires it, the officials said.
Anti-aircraft missile batteries are in place, while jets and helicopter gunships patrol the Normandy no-fly zone, French Air Force spokesman Captain Frederique Solano was quoted as saying.
He said Mirage fighter jets could be ordered to shoot down any aircraft ignoring warnings.
"The ultimate recourse would be, on the orders of the prime minister (Jean-Pierre Raffarin), to open fire on the aircraft," he said.
The French Navy will be patrolling a maritime exclusion zone.
Under the Vigipirate plan, demonstrations have been banned for a week in central Paris, including the Elysee presidential palace and the US embassy near Place de la Concorde.
The main commemorative events will be held on 6 June.