The pledge had been that the river would be transformed from a waterway frequently laden with sewage to one clean enough to swim in. Authorities spent 1.4 billion euros ($1.5 million) modernising the capital's antiquated sanitation works.

But the torrential rains that soaked the Games' opening ceremony and the first day of sport raised the levels of bacteria in the river beyond safe levels, forcing the repeated postponement of the men's triathlon until Wednesday and raising questions over the efficacy of the works.

"Here we are!" President Emmanuel Macron exclaimed on X. "In the space of 4 years, we've achieved something that hasn't been possible in 100 years: the Seine is now swimmable."

The women were the first to take the plunge in a triathlon -- which includes swimming, cycling and running -- that would be won by home favourite Cassandre Beaugrand . Soon later, it was the men's turn to leap off the floating pontoon underneath the Alexandre III bridge, near Napoleon's tomb.

The ambitious infrastructure works had included digging new pipes to houses and houseboats that once pumped raw sewage into the river, additional treatment plants and a cavernous reservoir underneath the capital's Gare d'Austerlitz.

Able to hold 46,000 cubic metres of runoff water during a storm, it was designed to prevent the runoff water mixing with untreated sewage and flowing into the Seine.

Wednesday morning's test results showed that the concentration of the E.Coli and Enterococcus faecalis bacteria had fallen below the European safe limit thresholds at all sample sites.

Athletes and organisers had disliked plan B: convert the triathlon to a duathlon. Following her hard-fought win, Beaugrand downplayed any concerns about the water quality.

"We've swum in worse," she said.

GREEN LIGHT

Olympians last swam in the Seine in the 1900 Paris Olympics. Former Paris Mayor Jacques Chirac in 1988 promised he would swim in the river "in the presence of witnesses", but his plunge never materialised.

Few had more at stake in the Paris 2024 river swimming than mayor Anne Hidalgo, whose campaign to transform the capital into a greener urban landscape has frequently drawn the ire of Parisians.

Earlier this month, Hidalgo herself dove into the Seine in a stunt designed to show confidence in the water's cleanliness, but which itself was postponed due to pollution levels

"We won our bet! The pollution has been cleaned up," the Paris mayor told reporters after the triathlon had begun.

Wednesday's races were given the green light despite rain overnight.

Organisers make the call based on analysis of river samples taken the previous day at 5 a.m. and after discussions with experts on weather, according to Paris 2024.

Focus now turns to the marathon swimming later on Aug 8 and 9, shortly before the Games wrap up. Unable to afford postponements, organisers have said those events would be moved to the rowing venue if the river is too dirty.

($1 = 0.9229 euros)