Security net over Seine

Central Paris turns into fortress as metal barriers seal off both river banks.

On the water, perched on rooftops and operating Al-augmented cameras, French security forces mounted a colossal operation to prevent an incident ruining the opening ceremony of the Paris Olympics on the river Seine last night.

The figures tell only part of the story of the efforts made to protect the river parade, the first time a Summer Olympics has started outside a stadium.

About 45 000 police and paramilitary officers were on duty with 10 000 soldiers and 20 000 private security guards.

The total area that needed securing measured more than six kilometres along the Seine and contained around 300 000 ticketed spectators, as well as hundreds of thousands of residents and tourists in overlooking buildings.

A no-fly zone 150km wide around Paris was enforced an hour before the ceremony started at 7.30pm.

This grounded or diverted all aviation at one of Europe's busiest airport hubs.

"This opening ceremony is the most extraordinary thing a country can do," said Interior Minister Gerald Darmanin on Wednesday.

It comes at a time when France is on maximum alert for terror attacks.

"Security of 100% does not exist," said Frederic Pechenard, a former head of the French police force.