French President Emmanuel Macron says the Paris Olympics opening ceremony planned on the River Seine could be shifted to the Stade de France if the security threat is deemed too high.

France is on high security alert ahead of the Paris Olympics and Paralympics, which are expected to draw millions of visitors to the country.

Security concerns are notably high for the planned opening ceremony, which involves boats carrying athletes along the Seine on a six kilometre parade and huge crowds watching from the embankments.

Speaking to French media, Macron said France's law enforcement forces will be mobilised at an exceptional level for the security of the open-air event.

"But if we think there are risks, depending on our analysis of the context, we have fallback scenarios," Macron said. "There are plan Bs and plan Cs."

The July 26 event is set to be the first Olympic opening ceremony held outside a stadium setting. About 10,500 athletes will parade through the heart of the French capital on boats on the Seine along the route ending in front of the Trocadero.

To limit security risks, Macron said organisers could decide to shorten the itinerary of the parade on the Seine, and even to "repatriate the ceremony to the Stade de France" for a more conventional opening event.

Organisers had originally planned a grandiose opening ceremony for as many as 600,000 people, most watching free of charge from riverbanks.

But security and logistical concerns have led the government to progressively scale back its ambitions. Earlier this year, the overall number of spectators was reduced to around 300,000.

The French government also decided that tourists won't be given free access to watch the opening ceremony because of security concerns. Free access will be invitation-only instead.

Macron insisted that, for now, plans for the opening ceremony remain the same.

"It's a world first. We can do it and we will do it," he said.

France has repeatedly been hit by deadly Islamic State attacks, including the Bataclan theatre massacre in 2015 in which extremists opened fire on concert-goers and held hostages for hours. French troops have also fought against Islamic extremists in the Middle East and Africa.

Last month, the French government increased its security alert posture to the highest level after the deadly attack at a Russian concert hall and the Islamic State's claim of responsibility.

Macron said that security perimeters will be installed "days, even weeks" before the opening ceremony. He added that road traffic in the high-security zone will be brought to a standstill, and that French authorities will use "drone systems, coding, cyber protection," in their safeguarding efforts.

Answering a viewer who expressed concerns about her son attending the opening ceremony, Macron said "If there's one place where your son will be safe, it's here."

"There are always risks in life," he said. "And we see it every day, unfortunately. But we've given ourselves the means to do it."