Britain’s first gold medal of the Paralympic Games went to a teenager, and in world record time, as Poppy Maskill cleared the field in the pool to win the S14 100m women’s butterfly, a moment she described as “unreal”.

The 19-year-old from Middlewich, competing in her first Paralympics, led from the front and held off the challenge of Hong Kong’s Yui Lam Chan to finish in 1min 03sec, shaving 0.33sec off the previous record. Unusually, that record had been set only four months ago by another British teenager, 17-year-old Olivia Newman-Baronius, who finished in fourth place on Thursday.

“I was just hoping and just doing my hardest and see what happens, so that’s what I did,” said Maskill, who will race again in the S14 backstroke, freestyle and SM14 individual medley during the Paris games. “It gives me more confidence [to win this race],” she said. “But we’ll see what happens in the rest of the Games.

“Being here at my first Paralympics feels unreal. My mum, dad, sister and nan are here and I can’t wait to see them. It feels so weird to be the first gold medallist for ParalympicsGB. After this I’ll probably just call my parents and then go to bed.”

Maskill and Newman-Baronius, who compete in the classification S14 for athletes with an intellectual impairment, are part of a new generation of British para-swimmers in the pool with 15 debutants in the 26-strong squad. Fellow debutant William Ellard claimed a silver medal in the evening session, in the S14 100m men’s butterfly.

A comparative veteran, 27-year-old Tully Kearney doubled the gold tally in the last race of the night as she upgraded her Tokyo silver medal to claim the S5 200m freestyle title. Kearney swam a composed race to hold off the challenge of Ukrainian Irina Poida and afterwards spoke of her joy after recovering from a head injury and health issues that at one point looked like keeping her out of the Games.

“I’m just absolutely ecstatic. A bit speechless,” she said. “I really wanted redemption for Tokyo. I was never happy with that silver, so to go and get gold in the 200m freestyle was just incredible. I was really happy with it.

“It’s been really hard with the concussion and then the last three, four months, really hard with mental health issues. Even a few weeks ago, we weren’t sure whether I was going to come out and compete or how many events I’d be able to do. So, we’re really grateful to the team for all the support they’ve given me in ­enabling me to come here and do this.”

A sold-out crowd at the La Défense Arena witnessed two world records on the night, the second struck by China’s Chen Yi in the women’s S10 50m freestyle, while there were two Paralympic records broken too as 15 events were decided in the first evening session. But the main event for the French fans came in the first race, when local favourite Ugo Didier won gold in the men’s S9 400m freestyle.

Trailing the Italian Simone Barlaam with two lengths to go, Didier kicked on magnificently to push beyond the Italian and finish a second clear. Didier – a civil engineer who was born with club feet – also became the first French para-athlete to have a cardboard cutout of his head jiggled by the crowd.

One final performance of note came from the Brazilian known as Gabrielzinho. The only armless swimmer in the men’s S2 100m backstroke final won a tightly contested race and won the affection of the crowd in the process. He acknowledged applause on the medal podium with a bow, as he accepted one of three golds he has promised to bring home to the city of Juiz de Fora.