The Paris Olympics are set to open in a spectacular and unprecedented ceremony on the river Seine on Friday but hours before the show France's rail network was paralysed by what officials said were acts of sabotage.

The parade on Friday evening will see up to 7,500 competitors travel down a six-kilometre (four-mile) stretch of the Seine on a flotilla of 85 boats.

But early Friday, French rail operator SNCF said the country's high-speed network had been targeted by "malicious acts" aimed at paralysing the system.

Transport Minister Patrice Vergriete said it was an "outrageous criminal act" and Sports Minister Amelie Oudea-Castera described the attacks as "downright appalling".

French security forces have warned for months of the danger of attacks aimed at destabilising the Games.

Suspicion will fall on Russia, whose team is banned from Paris in response to the Kremlin's invasion of Ukraine. France is seen as a particular target due to its support for Kyiv.

However, far-left groups within France have a history of targeting the rail network with arson attacks.

Compared to the Covid-blighted 2020 Tokyo Olympics, which were delayed by a year and opened in an empty stadium, the Paris opening ceremony will take place in front of 300,000 spectators and an audience of VIPs and celebrities from around the world.

Rain threatened to blight the ceremony after forecasters said there was a 70-80 percent chance of rain during the ceremony.

President Emmanuel Macron told a pre-Games dinner for heads of state and government: "Tomorrow you will have one of the most incredible opening ceremonies."

The line-up of performers is a closely guarded secret but US pop star Lady Gaga and French-Malian singer Aya Nakamura -- the most listened-to French-speaking singer in the world -- are rumoured to be among them.

It will be the first time a Summer Olympics has opened outside the main athletics stadium, a decision fraught with danger at a time when France is on its highest alert for terrorism.

For months, organisers have been dogged by questions about whether they would need to scale back or move the procession, but they had insisted throughout that there was no Plan B.

- 'Difficult to secure' -

A huge security perimeter has been erected along both banks of the Seine, guarded around the clock by some of the 45,000 police and paramilitary officers who will be on duty on Friday evening.

Another 10,000 soldiers are set to add to the security blanket along with 22,000 private security guards.

"Without any doubt, it is much more difficult to secure half of Paris than to secure a stadium, where you have 80,000 people and you can frisk them and send them through turnstiles," Frederic Pechenard, an ex-director general of the French police, told AFP.

Police snipers are set to be positioned on every high point along the route of the river convoy, which is overlooked by hundreds of buildings.

An assassination attempt on US presidential candidate Donald Trump on July 13 has focused minds.

Armed officers will also be on the boats, a security source told AFP.

The Israeli and Palestinian teams will be given extra protection, with the tensions caused by Israel's offensive in Gaza already spilling into the Games.

Organisers will be on guard against fresh protests after the Israeli football team's first match on Wednesday was marked by the waving of Palestinian flags and the booing of the Israeli anthem.

The opening ceremony is likely to define the mood for the rest of the July 26-August 11 Games, which organisers have pledged will be "iconic".

Around 3,000 dancers are set to perform from the banks of the river and nearby monuments, including Notre-Dame cathedral, in a show that will promote diversity, gender equality and French history.

The landmarks and architecture of the City of Light, one of the world's best-loved destinations, is set to feature as a backdrop both to Friday night's show and much of the sport afterwards.

Paris's vision is for a more cost-effective and less polluting Olympics than previous editions, with competitions set to take place at historic locations around the capital.

For scheduling reasons, some events have already started, including the football, rugby sevens and archery -- the latter taking place in front of the golden-domed Invalides, the final resting place of Napoleon.