French youngster Leon Marchand produced a dazzling performance to shatter American great Michael Phelps' long-standing 400 metres individual medley record on the opening day of swimming events at the World Aquatics Championships on Sunday.

Phelps' mark of 4:03.84 set at the 2008 Beijing Olympics was his last remaining individual world record but it fell to the 21-year-old Marchand who started superbly and clocked 4:02.50 to beat Carson Foster who took silver.

"That was insane, one of the most painful things I've done. It was amazing to do it here and the time is crazy," Marchand, who is coached by Phelps' former mentor Bob Bowman said.

"The best is yet to come," added Marchand, who collected his medal at the victory ceremony from Phelps.

There was cheer for home fans at the Marine Messe Fukuoka Hall as Japan's Daiya Seto took the bronze finishing more than six seconds behind Marchand.

Titmus shines

Earlier, Olympic champion Ariarne Titmus set a world record by blazing to the finish line in a battle royale for the women's 400m freestyle title on a great day in the pool for Australia.

Titmus eclipsed the mark of 16-year-old Canadian prodigy Summer McIntosh set at her national trials earlier this year, finishing in three minutes, 55.38 — a stunning 3.35 seconds ahead of silver medallist and 2022 champion Katie Ledecky.