The Nanjing Yangtze river bridge is four lanes wide and four miles long, a monument to Maoist endeavour clogged with the traffic of China’s economic boom. And every weekend, at one of its two towers, you can see Chen Si. He is 42 years old, with spiky black hair, a rasping cough from cheap Nanjing-brand cigarettes, and a baseball cap bearing the slogan ‘THEY SPY ON YOU’. Around his neck is an oversized pair of binoculars, through which he watches the crowds unceasingly. In the past six years, according to his blog, he has saved 174 people from suicide.

Mr Chen used to be a functionary at a transport company. He read one day in a newspaper that Mao’s famous bridge was now a suicide spot, and shortly afterwards began to go there whenever he wasn’t working and pull down suicidal people as they attempted to climb the railings. He has become celebrated — a concerned citizen taking a stand in a country with 200,000 reported suicides a year, and few good ideas about how to stop them. In Guangzhou, 700 miles south of here, officials tried to stop people jumping to their deaths from a steel bridge by having the structure smeared with butter.

But as I looked at him standing sentry at the south tower, Mr Chen’s project seemed almost as absurd. How could he possibly pick out the suicidal on a four-mile-long bridge? And was he serious with those binoculars, especially with visibility reduced to 50 yards or so in the storm? I introduced myself, and he waved me off. ‘Not now,’ he snapped. ‘I’m working.’ Then the binoculars shot up to his eyes.

Eventually, Mr Chen did talk to me. He never told me what I really wanted to know — why was he out here? Why had he reduced his life to this one repeated act of standing guard? Perhaps because he didn’t know himself. Instead he told me about his hair, which he has had to start dyeing after stress turned much of it grey, and his recurring nightmare, in which he runs to stop a suicide and always arrives a moment too late. He can spot a suicide case, he has said, from the way they walk: ‘From the crowd of people, I’ll single out those who look depressed, those whose psychological pressure is great. Their way of walking is passive with no spirit and no direction.’

Chen blames China’s frightening suicide rate on the pace at which change has taken place. Families have fractured as young people travel far and wide in search of work, and the elderly are left alone. The single child policy means the responsibility of caring for ageing parents is not shared with siblings.

‘When I was young, we had little to eat, but the suicide rate was low,’ said Chen. ‘Now we all have meat but people cannot deal with the stress.’ Suicide is now the leading cause of death for Chinese men between the ages of 15 and 34.

After our chat, Chen fired up his rackety moped and weaved between the walkers on the pavement to the north tower. On the pavement, he has spray-painted messages containing his mobile phone number; from the railing, he has hung banners reading ‘Value Life Every Day!’

In his absence, a man in green overalls appeared at the south tower, lurching and seemingly drunk, and began climbing the railing, 150 feet over the Yangtze. A bystander tackled him and dragged him to the ground. A crowd gathered, confusion built, and the man began to cry. And then someone — perhaps they checked one of the messages sprayed on to the pavement — called Mr Chen’s mobile.

When Mr Chen returned, the man launched into his story — former soldier; dead mother; sick father who’d also been a soldier; job in a petrol station that didn’t pay enough to keep the two of them — and Mr Chen cut him off. ‘I should punch you in the face,’ he said. ‘You call yourself a family man, a son, Chinese? If your father hadn’t been in the army, and if you didn’t try to kill yourself just now, I’d punch you. You’re not thinking — or are you just shirking your responsibility? I really would like to punch you now. Hand over your ID.’

The man, flummoxed, reached into his pocket and handed over the card, which gave his name as Fan Ping. Mr Chen made a show of studying it, then handed it back and asked what Fan had been thinking, coming up here like this. ‘Yes,’ he said, dismissively, to the answer. ‘We all have our troubles.’

They continued to talk, however, and Mr Chen’s tone shifted slowly from outrage to fraternal concern. After they had talked for a good while, he moved in closer and clasped Fan Ping’s hand, and didn’t let go until he had dragged him to the south tower bus stop. He arranged for him to come to his office first thing on Monday morning, writing the address on a scrap of paper and stuffing it into his pocket. He punched Fan Ping’s mobile number into his phone. ‘I promise you that there’s nothing we can’t fix,’ he said, ‘but first we have to get you off this bridge.’

Mr Chen’s vigilance doesn’t end when he leaves the bridge. He watches over the men and women he saves, telephones them regularly and thinks up ways to help solve their many problems. With each life he saves, his own becomes more complicated, more burdened with worry. His wife complains about the hours he spends on the bridge and the shaken survivors he brings home for her to feed. ‘But what else would I do with the people I save?’ he has asked.

Every year, around Christmas, a few of the people that Mr Chen has managed to talk off the bridge meet back there to celebrate their new lives. As part of the ceremony, they calculate their new ages from the date of their salvation. No one at the party is older than six.

But Mr Chen’s blog often seems to be more about the people he has failed to save. A husband and wife who jumped hand in hand. A man dressed in black, whose body was sucked away by the current as a boat tried to reach him. Another man who fought after he was pulled from the railing, bit his tongue in half and nearly bled to death on the pavement.

The blog reads as a collage of dark news, in which Fan Ping’s story would barely be distinctive enough to register. ‘Middle-aged man jumped off bridge where the body fell to the flower bed: died on the spot… Speaking in northern accent, man gave me a cigarette, said: “Alas! Wives and children”… A woman in the southeast fort jumped in riverbed, dead on spot… Next to statue at southwest fort, man died jumping to concrete, one leg thrown from body, only blackened blood left behind. Meaningless life!’

As I left the Nanjing Yangtze River Bridge, the bus containing a contrite Fan Ping was pulling away, and Mr Chen once more raised his binoculars. An interminable rain slithered down from the sky but Mr Chen stayed at his post by the south tower, monitoring the streams of students, businessmen, lovers and pensioners walking over the bridge, trying to work out who would be next.