Leon Marchand said he had goosebumps after romping to France's first Olympic swimming gold since 2012 in front of a packed and raucous Paris crowd on Sunday.

Marchand obliterated the rest of the field in the 400m individual medley final in clocking 4mins 02.95secs, nearly six seconds clear of Japan's Tomoyuki Matsushita in second, with American Carson Foster third.

It was the second-fastest 400m IM time ever and gave most of the spectators at La Defense Arena what they came for on the second night of competition.

The 22-year-old was greeted with a massive roar when he entered the arena and the noise did not let up until long after he had touched the wall in an Olympic record time.

"I had goosebumps on the podium," said Marchand, who will also swim the 200m medley and 200m butterfly.

"I felt really proud to be myself and to be also French tonight. It was an amazing time for me and I was really living it in the moment, so that was pretty cool."

Marchand said he took a call from French president Emmanuel Macron after the race.

"He told me that he was watching the final with his whole family," said Marchand.

"Everyone was screaming on the phone. It was kind of funny and I was very grateful for that phone call."

Marchand was heavy favourite after demolishing Michael Phelps' 15-year-old world record in a phenomenal 4:02.50 swim last year.

With American defending champion Chase Kalisz failing to reach the final and with the packed crowd roaring him on, Marchand was half a body length clear after the opening butterfly leg.

But it was a sensational backstroke 100m that left his rivals trailing.

Trained by Phelps's old coach Bob Bowman, his win made him France's first pool gold medallist since the London Games.

At the pandemic-delayed 2021 Tokyo Olympics, they took home just one swimming medal, Florent Manaudou's silver from the 50m free.

"I was trying to focus on myself, but it's really hard when 15,000 people are cheering for me," said Marchand.

"I did well in trying to use this energy to swim as fast as possible."

Marchand said he would go to bed early as he prepares for the rest of his programme at the Games.

"Going forward, I know that I prepared well and I know that I'm ready for some high-intensity races."