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Paris mayor Anne Hidalgo and head of the Olympics committee, Tony Estanguet, took a dip in the Seine on Wednesday, in a bid to prove that a costly clean-up had made the river ready for athletes to use at the upcoming games.

Hundreds lined up on nearby quays and perched on bridges as the pair hopped into the historic artery from a temporary dock erected near the town hall to shouts of “bravo”. 

“The swim was amazing, the water was great, a bit cold but not too much,” said a beaming wetsuit-clad Hidalgo. “Making the Seine swimmable for residents will be an important heritage of the games.”

“Today we sent an important message to athletes — the triathlon and marathon swimming events will be able to take place in the Seine,” Estanguet added. “It was a daring idea to hold the events in the river, and it has paid off.”

The stunt occurred amid fears that unseasonably bad weather would render the Seine far too dirty for the events to take place. Water levels are four times higher than normal for summer. Last year several trials were cancelled over unsafe levels of bacteria.

In early June, bacteria levels remained unsafe. While recent tests, done daily, have shown acceptable levels of E. coli and other bacteria, if it rains heavily during the games, it could push the bacteria levels above the 900 colony-forming units per 100 millilitres limit set by the World Triathlon Federation as safe for competitions.

The organising committee for the Paris Games has no back-up venue lined up, although they can change the days of the two events.

The Paris region and the national government have spent about €1.4bn on five major engineering projects to clean up the Seine and Marne rivers ahead of the games, which open on July 26 and run until August 11. 

Additional water treatment plants were built, while hundreds of houseboats and 20,000 homes and buildings along on the Seine were linked to the sewage system for the first time.

A 700-metre underground tunnel and large storage tank were built along the right bank of the river, at a cost of about €90mn, to capture overflow from the French capital’s antique sewers and prevent pollution during periods of heavy rainfall when run-off dumps into the Seine.

Wearing a blue striped swimming costume issued by the town hall, Samuel Colin-Canivez, the lead engineer for the Paris water system, expressed pride and excitement as he waited to jump in the river.

“It’s the culmination of years and years of work,” he told the Financial Times.

Under a blue sky, Hidalgo and Estanguet were accompanied on the swim by dozens of officials, athletes and engineers who worked on the project, and even a few journalists.

The decision to hold the watersports events in the Seine is one of many daring choices made by the Paris Olympics organisers.

For the first time, the games are being held mostly in temporary venues built in the historic centre of the French capital — beach volleyball will take place at the foot of the Eiffel Tower; equestrian events at Versailles and the inaugural breakdancing competition at Place de la Concorde.

The opening ceremony is an extravaganza of hundreds of boats carrying athletes along a 6km route on the Seine. Security will be a major concern, with hundreds of thousands of spectators lined up on the quays and watching from balconies of the historic buildings.

Swimming in the Seine was banned in 1923 because of pollution, with heavy rains regularly bringing flows of rubbish and plastic.

Politicians have long promised to bring it back. Former president Jacques Chirac vowed to get it done when he was Paris mayor in the 1990s.

Hidalgo has billed the Olympics as a catalyst for the opening of 20 swimming spots by 2025, which has become hotter in summer due to global warming. “Giving the Seine back to residents is part of adapting our city to climate change.”