Britain's Adam Peaty, Swede Sarah Sjostrom and Katie Ledecky of the USA all smashed world records in the 100m breast stroke, 100m butterfly and 400m freestyle respectively in the early hours of Monday morning.
Peaty ended his country's 28-year wait for a men's Olympic swimming gold with the nation's first medal of the Rio Games.
The 21-year-old emulated Adrian Moorhouse, who won the same event in Seoul in 1988.
Peaty broke his own world record in winning in 57.13 seconds and also broke the British medal duck after near misses for fellow swimmer Hannah Miley and fencer Richard Kruse.
"I touched the wall and looked around and thought 'where is everybody?' Peaty told reporters.
Defending champion Cameron van der Burgh of South Africa took the silver in 58.69 and Cody Miller of the United States won the bronze in 58.87.
Sjostrom also obliterated her own world record to become the first Swedish woman to win an Olympic gold medal for swimming.
Penny Oleksiak of Canada took silver and Dana Vollmer of the United States, the defending Olympic champion, earned bronze.
Sjostrom, three times world champion in this event, set a time of 55.48 seconds to break her previous mark of 55.64 seconds, set in Russia a year ago. It was the first Olympic medal for the 22-year-old Swede, competing in her third Games.
Sjostrom will also race in the 50, 100 and 200m freestyle.
Ledecky, the latest star of American swimming, followed their lead by ripping up her own 400 freestyle world record. Britain's Jazz Carlin took the silver medal, and Leah Smith of the US won bronze.
Ledecky led from start to finish to set a new mark of 3:56.46 seconds, bettering the 3:58.37 she set in Australia two years ago.
The victory was the first stage of a rare treble she hopes to complete along with the 200m and 800m.
"It's pure happiness," the 19-year-old from Maryland said. "I wanted the first 200 to hurt as little as possible and I really felt like I could build into it and really explode that last 50."
In a reminder of the doping controversies that dogged the build-up to the Olympics, there were loud boos for Russian breaststroker Yulia Efimova and the men's relay team.
Efimova, who has served two doping suspensions and , succeeded in an appeal last week against being banned from Rio.
She was one of a number of Russians who argued successfully that excluding them from the Olympics would be punishing them again for the same offence.
She qualified second for Monday's 100 breaststroke final, 0.02 seconds behind Lilly King of the United States.
In a reminder of the doping controversies that dogged the build-up to the Olympics, there were loud boos for Russian breaststroker Yulia Efimova and the men's relay team.
Efimova, who has served two doping suspensions, succeeded in an appeal last week against being banned from Rio and was heavily criticised by Irish swimmer Fiona Doyle following the afternoon's heats.
She was one of a number of Russians who argued successfully that excluding them from the Olympics would be punishing them again for the same offence.
She qualified second for Monday's 100 breaststroke final, 0.02 seconds behind Lilly King of the United States.
