Swimming in the Seine is becoming an increasingly popular tourist attraction – as well as a must-do for Parisians themselves.

Thousands of people have enjoyed a dip in the French capital's river since the opening to the public of three bathing sites last month, a first in over a century.

The swimming areas are expected to get even more crowded as a heatwave is to affect the Paris region on Tuesday.

The capital has been placed under “high vigilance” for hot temperatures by national weather service Meteo France - with up to 38 degrees Celsius (100 degrees Fahrenheit) expected.

At the Grenelle swim site, in the west of Paris, visitors are swimming lengths and sunbathing with a unique view on the Eiffel Tower.

A number of small fish near the surface appear to become used to the company of the swimmers.

The water quality is being tested daily to confirm it's in line with European regulations.

“Imagine that! Swimming with view on the Eiffel Tower and in pure natural water, clean, safe and with all these lovely people too," Constanze Martens, a tourist from Mexico, said.

"People actually like swimming for real and just chilling out. It’s a great mix of everything.”

On Monday, the water temperature in the Seine measured 22 degrees Celsius (71 degrees Fahrenheit).

Until the end of August, bathing sites are open for free at scheduled times to anyone with a minimum age of 10 or 14 years, depending on the location.

Swimming in the Seine had been illegal since 1923, with a few exceptions, due to pollution and risks posed by river navigation.

The opening of bathing sites has been made possible following a 1.4 billion euro ($1.5 billion) cleanup project that made it suitable for Olympic competitions last year.

Some, like Australian visitor Thurkka Jeyakumar, first showed scepticism at the idea of swimming in the Seine, often citing the river’s murky color and bacteria issues.

In the end, Jeyakumar gave it a try because she lost a bet.

“The result of the bet was that when I came to Paris, I had to swim in the Seine because I was saying that I would never swim in the Seine!"

“For the moment, I have to say that it was much nicer and cleaner than I thought it would be," she said. "So the bet worked out for the better.”

AP Video shot by Nicolas Garriga