An epic women’s triathlon – and an epic result for France and Paris. The crowds were 10-deep along the banks of the Seine and how they roared as Cassandre Beaugrand, who was born in the city, powered away on the final lap to take a thrilling gold medal. Not far behind was Britain’s Beth Potter, who took a gritty bronze medal in conditions that were brutal even by the standards of this tough sport.

The water was treacherous. And the bike hugely slippery. Afterwards the 27-year-old Beaugrand credited a move to Loughborough two years ago with helping her cope with the wet conditions as she won in 1hr 54mins and 55secs.

Related: Alex Yee delivers thrilling late surge to secure gold for GB in men’s triathlon

“I just can’t believe it,” said Beagrand, after winning France’s sixth gold medal of the Games. “I want someone to pinch me right now. I moved to Loughborough so I am used to riding in the rain now. It took me out of my comfort zone. The English people helped me a lot, so thanks to them.”

Behind her the Swiss athlete Julie Derron took silver, six seconds back, with Potter third a further nine seconds behind. The men’s race that followed was the first without a Brownlee brother since 2004, but he was part of the team that helped coach Potter, who was delighted with her result.

“I am just so happy,” said Potter, who switched to triathlon after competing for Team GB over 10,000m in Rio in 2016. “I was going for the gold but Cassandre and Julie were just too good. I have come a long way in eight years.”

For months British triathletes had prepared for the pollution in the Seine by using probiotics and mouthwash and by scrubbing their tri-suits down with Hibiscrub disinfectant. But when the race began they found themselves facing a dastardly new problem: the strong and uncertain currents along the 1500m swim under the Pont Alexandre III bridge.

With no practice allowed in the Seine on Sunday and Monday because of the high E coli levels, there was no way to learn its peculiar whims and wiles. However, the Tokyo 2020 gold medallist Duffy quickly worked out where to best position herself as she pulled herself out of the water first in 22:05.

Still, the two Britons were not far behind, with Potter in fifth, 18 seconds back, and GeorgiaTaylor Brown in 10th place, 36 seconds back. It was then up the 36 stone steps up the Pont Alexandre III bridge and on to the bike.

It soon transpired that the 40km bike course was even more devilish than the swim. The cobbles would have been tricky enough. But the heavy overnight rain had made the paint on the zebra crossings especially slippy. It led to crashes galore as the back wheels of bikes suddenly gave way without warning. Pity poor Lotte Miller of Norway, who crashed twice.

For the first of the seven laps on the bike, Duffy kept her lead. But by the end of the second lap a group of 10 had formed that included Potter and Taylor-Brown, as well as France’s two big stars, Beaugrand and Emma Lombardi. But that group was reduced to nine just before for the run after a crash took out the German Lisa Tersch.

Derron was off the bike first in 1:21:43, with Potter third out of transition as she started her run. As a 10,000m runner she would have fancied her chances as she began the four laps of 2.5km that made up the 10km run.

Within a few hundred metres there was a leading group of six triathletes as they went up the Champs-Elysees, with Derron first and the two British women just behind her. Duffy was the first to slip back. Then Taylor-Brown began to drift off. By the end of the first lap there were four women left chasing the three medals.

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The huge crowds, that were six or seven deep on the course, urged the two French athletes on as the temperatures rose to 25c with humidity at 90%. During the second and third laps, the gutsy Derron kept pushing but the three behind kept responding.

At the bell, it was Derron leading, Beaugrand second, Potter third, and Lombardi fourth. But with barely two metres separating them. But Beaugrand always looked strong and when she made her move halfway through the final lap it proved decisive.