Yang, a retiree, told AFP that for him it was a way to "enjoy the great outdoors" in the heart of the Chinese capital.

He said he began "winter swimming" in the icy waters of Shichahai lake in 2007 as a form of exercise.

"Natural bodies of water have better air quality than swimming pools," he added.

He is usually joined by the same group of people who add a frigid lap in the lake to their daily routine, although more health-conscious young people have joined recently, said Yang.

Zhang Xin, a 40-year-old freelancer, took up winter swimming four years ago.

"I used to catch colds all the time," he said. "Through running and swimming, one can get stronger and improve health."

A handful of passers-by stopped to watch as Zhang let out a few rallying cries before splashing into the lake.

He stayed in the water for around a minute, completing one lap with freestyle and butterfly stroke.

"You can't stay in for too long," Zhang told AFP, calling the winter dip an "extreme sport".

Bus driver Gu Yueping, now in his 11th year of winter swimming, squeezed in an icy lap during his midday break.

"Once you're in the water, it's like a full-body acupuncture massage," the 46-year-old said.

Since taking up winter swimming, he said full days behind the wheel have become less exhausting.

"Driving a bus, you're sitting all day," said Gu, who used to deal with body aches and leg pain.

"Now driving for work doesn't feel that tiring."

Long-time swimmers say warming winters, spurred by climate change, mean the lake is no longer as cold.

"When we first started swimming it was always around -10C during the day," but recent winters have felt "milder", said Yang.

He greeted a steady stream of swimmers, mostly of retirement age, with some jogging on the spot to warm up for the frozen plunge.

For bus driver Gu, practice helps build tolerance.

"Your skin forms a memory. And you're no longer afraid," he said.

"Winter swimming is this: be uncomfortable for a moment to feel good for a whole day."