PARIS: Paris Mayor Anne Hidalgo took a long-awaited swim in the murky waters of the Seine on Wednesday amid joy and relief that the river is finally clean enough for outdoor Olympic swimming events later this month.

Wearing goggles and a wet suit, the 65-year-old city leader swam breaststroke under bright sunshine before immersing her face and switching to front crawl, covering around 100 metres (yards) downstream.

"It's wonderful, very, very pleasant," Hidalgo told reporters of the greenish water. "It's fresh but not cold."

She was joined by senior local officials and chief organiser of the Paris Games, Tony Estanguet, a gold medal-winning canoeist who has been sweating on the state of the Seine ahead of the opening ceremony next week.

"Today is a confirmation that we are exactly where we are meant to be," Estanguet said. "We are now ready to organise the Games in the Seine."

The swimming leg of the Olympics triathlon is set to take place in its waters on July 30-31 and August 5, followed by the open-water swimming on August 8-9.

Despite an investment of 1.4 billion euros (US$1.5 billion) to prevent sewage leaks into the waterway, the river had been continually failing water quality tests until the start of this month.

Wednesday's dip in front of more than 150 journalists was intended to demonstrate that the river was ready, but also to underline what is meant to be one of the key legacy achievements of Paris 2024.

Hidalgo plans to create three public bathing areas in the Seine for the city's residents next year - a century after swimming was banned - while fish and other organisms are also returning to the waterway in greater numbers.

Elsewhere on Wednesday, French police said they had arrested an 18-year-old suspected far-right extremist in the east of the country after he made threats against the Games in a Telegram group.

Security forces are on their highest alert ahead of the Games, with an alleged plot by a teenage Chechan to attack an Olympic football match thwarted in May.

Hidalgo had originally planned to swim last month, but had to delay because water tests showed that E.Coli bacteria - a key indicator of faecal matter - were sometimes up to 10 times higher than authorised limits.

Her swim had sparked jokes and memes on social media, with one viral AI-generated image showing her looking like the wrinkled Gollum character from the "Lord of the Rings" movies after emerging from the water.

A short-lived protest group posting under the hashtag #jechiedanslaSeine (#IshitintheSeine) had also encouraged people to empty their bowels in the river upstream.

President Emmanuel Macron, who had promised to join the Seine bathers, was a notable absentee on Wednesday as he is occupied by a political crisis caused by his decision to call snap parliamentary elections last month.

The locations chosen for outdoor swimming at the Olympics have caused difficulties in the past, notably ahead of the 2016 Rio de Janeiro Games and those in Tokyo in 2021.

French authorities have invested in new water treatment and storage facilities in and around Paris, as well as ensuring that thousands of homes and canal boats without waste water connections are linked up to the sewerage system.

"If it hadn't been for the Games and Tony Estanguet, we wouldn't have been able to do all this work, because the Games were an accelerator," Hidalgo explained.

Marc Valmassoni from campaign group Surfrider said cleaning up urban rivers was essential for the future when higher temperatures due to global warming mean city-dwellers will need swimming areas to cool off in.

"If the water courses are unsafe, it will put them in danger," he told AFP.

Major storms still overwhelm the Paris underground waste-water network, leading to emergency discharges of untreated sewage into the Seine.

The Olympic events remain weather-dependent and heavy rain on the eve of the triathlon or open-water swimming could lead to problems.

In the worst-case scenario, the swimming for the triathlon would be cancelled, while the open-water swimming could be moved from the Seine to a water course east of Paris.

Heavy rain in May and June have also disrupted preparations for the Seine-based opening ceremony, during which thousands of athletes are set to sail down the river.

Organisers have had to postpone rehearsals repeatedly because of the strength of the currents.

The Seine's flow is currently around 400 m3/second - many times above the usual level for this time of year of around 100-150 m3/s.