Summer McIntosh set the pool alight at La Defense Arena in Paris, with a commanding victory in the women's 400m individual medley.

The Canadian won in a time of 4 minutes, 27.71 seconds, ahead of American silver medallist Katie Grimes and her bronze-winning US teammate Emma Weyant.

McIntosh's triumph comes two days after taking silver behind Australian powerhouse Ariarne Titmus in the 400m freestyle on the meet's opening on Saturday.

In contrast, the men's 200m freestyle a closely fought battle between at least six swimmers in the field.

In the end, it was Romanian teenager David Popovici who powered to the gold medal on Monday, edging Britain's Matthew Richards by a fingertip.

The 19-year-old touched in 1 minute 44.72 seconds ahead of Richards who took silver.

American Luke Hobson took bronze in a frantic finish.

It was Popovici's first Olympic medal after missing out in Tokyo where he came fourth and he also became the first Romanian male to win an Olympic gold in swimming.

It was another gold for Italy as world record holder Thomas Ceccon won the men's 100m backstroke gold at the Paris Olympics.

China's Xu Jiayu took silver, with 2016 gold medallist Ryan Murphy of the US taking bronze.

China also won silver in the women's 100m breaststroke as Tang Qianting finished just behind Tatjana Smith of South Africa who added the 100 breast title to the 200m gold she won in Tokyo three years ago.

Ireland's Mona McSharry completed the podium in third place. (Agencies)