SLOW music fills the air as a middle-aged man practices tai chi and nine police officer students watch and try to mimic his movements.
The man is in a half-crouch, back straight, hands circling forward and backward, then he stands up, legs shaking a bit as he ends his demonstration at a police station.
"My legs are still not as strong as they should be because of my illness," says community police officer Xu Fangqian in Huangpu District, who is also a 55-year-old cancer patient.
While tai chi is a relaxing pastime for many people, for Xu it's a life-long passion, a passion that has both drained him and restored him.
"I hate tai chi and love it too," says Xu who has taught thousands of students about traditional Chinese quan, or shadow-boxing.
Xu says overwork as well as pouring all his energy into tai chi may have contributed to his diagnosis in 2004 of nasopharyngeal carcinoma and following torturous radiation therapy. The nasopharynx is located behind the nose and above the back of the throat.
He was teaching tai chi almost all the time when he wasn't working and many people sought him out for lessons.
Xu is tall and fit, though he tires easily now, and he has never smoked.
He still receives treatment but he has rallied, going back to work and teaching tai chi. He has been on the police force since 1990.
His tai chi journey started when he was nine years old, the last of seven children in a martial arts family. He first learned Shaolin boxing but since he was weak and undernourished, his coach said he should switch from powerful boxing to gentler tai chi, and so he did.
At that time, traditional tai chi was largely put aside and replaced with "Loyalty Figure Quan," expressing devotion to Chairman Mao Zedong.
"Still the core of tai chi has been preserved through all political movements," Xu says.
He practiced through the years with 13 teachers, finally become a shifu or master more than 20 years ago.
"One side effect of teaching is that there is little time for your own practice," says Xu, who never turns a student away.
In teaching he emphasizes the importance of self-cultivation and achieving inner peace. To maintain health, the traditional Chinese emphasis is on conserving qi, the mysterious vital energy circulating through the body and achieving inner harmony.
"That's why I always set rules for practice when any talk is about tai chi, and there's no talking behind another person's back."
When his cancer was diagnosed in 2004, Xu was "too stunned and dumbstruck to say a word," Xu recalls. That was the year the Huangpu District Tai Chi Association was established and Xu was elected president.
His life was disrupted but tai chi eventually helped him regain personal tranquillity and health.
Xu has many fans, some following him for as long as 10 years.
Every Tuesday morning he teaches a community class, which includes a visually impaired man. Many people were surprised that Xu took him as a student, but it was natural for Xu, who gives the man directions and physically helps him in certain postures.
"Xu views everyone the same," says student Lou Ruyu, who is in her 60s. "And he teaches us everything he knows without reservation."
Learning tai chi is a lifelong commitment, Xu says.
"There's a saying that 'For three years a man practices tai chi and for 30 years he practices to be a human being'," he says.
In three years' practice, one can achieve beautiful movements, but it takes another 30 years to elevate one's character through tai chi, he said.
A tai chi practitioner should not stop outside the door for 10 years, he said, emphasizing the importance of long practice and dedication.
"But some people rush to become teachers after only two years' practice themselves," Xu says, adding that many people want quick success and instant profit without putting in hard work.
He quotes some parents as telling him, "my child does poorly in school. Please teach him tai chi so he (or she) can make big money in the future."
As for himself, Xu says, "I will keep practicing tai chi until the lights go out."