Thousands of athletes are set to sail through central Paris on Friday during an unprecedented and high-risk Olympics opening ceremony that will showcase the country's hugely ambitious vision for the Games.

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

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

"Tomorrow you will have one of the most incredible opening ceremonies," French President Emmanuel Macron promised at a pre-Games dinner for heads of state and government at the Louvre museum on Thursday evening.

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 round-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, where nearly 40,000 people are estimated to have died, already spilling into the Games.

Organisers will be on guard against fresh protests on Friday evening 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.

- Scandal-hit Canadians -

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.

"The opening ceremony is a huge event and one that, arguably, sets the tone for the next 17 days," Hugh Robertson, the minister charged with delivering the 2012 London Olympics, told AFP recently.

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.

On Thursday, women's football took centre stage after a chaotic start to the sporting action in the men's football 24 hours earlier caused by a pitch invasion during an Argentina-Morocco game.

US gymnastics superstar Simone Biles, set to once again be one of the faces of the Games, got her first taste of the Bercy Arena as she trained ahead of the start of competition at the weekend.

Biles is strongly tipped to add to her haul of four Olympic golds at the Paris Games after a tumultuous campaign in Tokyo three years ago, when she pulled out of most of her events as she battled the disorientating condition that gymnasts call "twisties".