It is billed as the biggest open-air show on Earth – a spectacle so ambitious it will shut down a city centre and its airspace, mobilise 45,000 police and several army units and has taken two years to rehearse in secret.

When the Paris Olympics opening ceremony begins on Friday night, it will be the first time the theatrical curtain-raiser for the world’s biggest sporting event has taken place outside a stadium.

Instead of the traditional parade round a stadium track, more than 8,000 of the world’s top athletes will be transported by boat along 6km of the River Seine in a kind of sporting Armada, as more than 300,000 people watch from the bridges and riverbanks and police, frogmen and snipers stand guard.

While the athletes glide down the river, dancers, pop stars, tightrope walkers and acrobats will perform daring feats on water, rooftops, bridges and artificial islands using pontoons, floating pianos, helicopters and possibly even submarines, before a vast laser show finale is beamed from the Eiffel Tower.

More than 1 billion people are expected to watch live on TV and social media.

The idea of performing a show of this magnitude along a large stretch of river in a city under its highest terrorism alert was so outrageous that even Emmanuel Macron at first thought it was “a crazy and not very serious idea”. But this week at the Elysée, the French president said: “We decided it was the right moment to deliver this crazy idea and make it real.”

Exactly what will take place in the almost four-hour long show has been kept tightly under wraps, but as the decor, including floating skateparks, giant replicas of paintings and oversized gold tinsel began to be placed in and around the Seine, some clues were emerging.

In the early evening, delegations of athletes from more than 200 countries will be bussed from the Olympic village to a spot at the Seine in the east of Paris where they will board boats from specially built pontoons under the guard of the French military. The delegations will then sail side by side down the Seine, past Notre Dame cathedral and the Louvre towards the Eiffel Tower. The Greek federation will be first, followed by the Olympic Refugee team, then other nations, with the French boat coming last.

As the athletes sail, a show will unfurl around them, described by its organisers as revolutionary and irreverent, as well as full of surprises.

Historical monuments, rooftops, bridges, the sky and the water will provide simultaneous settings for overlapping dance, music, acrobatics and light shows that explore the history of Paris in all its diversity as well as its tradition of rebellion. Tightrope walkers will cross the river on high wires, the star French ballet dancer Guillaume Diop will perform on a rooftop and BMX freestyle bikers will backflip over the water. The French pop star Aya Nakamura, who faced a racist backlash earlier this year over the possibility that she would sing, will perform – and is expected to sing hits by the singer-songwriter Charles Aznavour. Céline Dion is thought to be singing Édith Piaf, and Lady Gaga is also expected to appear.

“When we watch Emily in Paris or [the film] Amélie we know it’s not quite the real Paris – we’re going to play with all those cliches, but we’re also going to challenge them,” said Thomas Jolly, the young director who has created the show. “Paris is also its vibrant youth, different cultures rubbing shoulders in the streets.”

He envisioned the show as a giant performance in 12 acts, culminating at the Trocadero in the shadow of the Eiffel Tower, where 100 heads of state will watch.

France was impressed by the humour of the late queen’s apparent parachute from a plane with James Bond during London’s opening ceremony in 2012, and one priority for the Paris show is to provide its own form of tongue-in-cheek and inventiveness, to disprove any stereotype of French haughty seriousness.

The historian Patrick Boucheron, who has advised on the script for the Paris show, emphasised the importance of the show exploring France’s diverse history and connection to the rest of the world.

Fanny Herrero, the screenwriter who created the global hit series Call My Agent, has worked on the ceremony’s storyline. She was shown in an early production meeting warning against any notion of French can-can dancers and people playing the accordion, calling instead for “invention and humour”. There are expected to be riffs on Paris as the city of love and light, and a sketch involving scores of French waiters.

Leïla Slimani, the Franco-Moroccan novelist, who was also involved in devising the show, said it would bring sparkle and excitement.

The Paris mayor, Anne Hidalgo, said: “It will be very joyous and happy – extremely creative and not very well behaved. It is about the joy of diversity, a city open to the world.”

More than 3,000 dancers and actors will take part. At the last minute, strike action over dancers’ pay and conditions has been averted, avoiding the spectacle of performers standing with raised fists in protest during parts of the show.

But despite the waterside pyrotechnics and pirouettes, France is under pressure to keep in mind that the real heroes of the show are the athletes themselves, with each nation’s sporting flag-bearer standing at the prow of a boat. “The athletes can’t just be the background, they are the stars,” one official said.