The American swimmer, who has 13 Paralympic Games gold medals to her name, won the first-ever race at the newly-formed Series by topping the podium in the women's 100 metres freestyle mixed classification event.

In her first competitive appearance since Rio 2016, Long, who competes in the S8 category, won the event with a score of 946 points, beating Sweden's S14 competitor Pernilla Lindberg and S12 swimmer Naomi Maike Schnittger of Germany.

Following on from her historic success, the 25-year-old then reigned supreme in the women's 400m, which was also mixed classification.

Long finished with a total of 968 points, which was enough to see her clinch gold in front of team-mate and S13 swimmer Rebecca Myers and Schnittger, who added to her collection with another bronze.

"It’s just great," said Long.

"I’m so ecstatic about the World Series and I think it brings a lot of awareness to the movement of the Paralympics. 

"To be able to come here and compete for Team USA, there’s nothing better."

The US enjoyed a fruitful opening day as Leanne Smith broke the S4 world record in the women's 50m butterfly, an event which is not on the World Championships or Paralympics programme.

Smith stopped the clock at 49.6sec as she ended with a total of 1,047 points.

Meyers clinched another silver while bronze went the way of Hungary’s Zsofia Konkoly.

Norway’s Paralympic and world title holder Sarah Louise Rung continued to demonstrate her prowess in the women’s 100m breaststroke as she sealed victory in the mixed classification race with 1,022 points.

Germany’s Verena Schott earned silver, while Lindberg sealed her second medal of the day with bronze.

The Netherlands’ Marije Oosterhuis won the 50m freestlyle S10, held as a single class race, in a time of 29.63, with Estonia’s Susannah Kaul and Hungary’s Bianka Papa claiming silver and bronze respectively.

In the men's events, Portugal's three-time Paralympian David Grachat was the winner in the men's 100m freestyle mixed classification, becoming the first man to stand at the top of a World Series podium.

Grachat, an S9 competitor, scored 884 points, one more than Denmark's Jonas Bastian Sorensen, who races in the S14 category.

Finland’s Leo Lahteenmaki was the recipient of the bronze medal.

Niels Mortensen's total of 871 points gave Denmark their first gold of the evening in the men's 100m backstroke.

The S8 swimmer finished in front of Belgium’s S14 swimmer Aymeric Parmentier and S8 athlete Phillipe Vachon of Canada.

Paralympic gold medallist Aaron Moores of Britain, an SB14 competitor, claimed the men's 100m breaststroke honours, fighting off competition from Sweden’s Karl Forsman, who races in the SB5 events, and Parmentier.

The men's action was rounded off by Lahteenmaki winning the men's 50m freestyle S9 in 27.28.

Grachat was another multiple medallist on day one as he secured silver, while Hungary’s Bence Mocsari was third.

The event in the Danish capital continues tomorrow.