Liu Dapeng has spent this summer the same way he spent the last: hunting perverts who target female passengers in crowded subway cars.

He is among dozens of plainclothes police officers who patrol Beijing's vast subway network each day to prevent the sexual assault or harassment of women, the instances of which peak in warm-weather months.

The clampdown campaign was launched in June last year by the Sihui police station in the south of Chaoyang district, and then rolled out citywide over the following two months, with about 20 stations allocating resources.

So far, 155 men have been fined or given five to 15 days in a detention house－an extrajudicial punishment mostly used for minor offenses－after being caught groping or molesting women on trains.

Liu, who was the first to suggest intensifying patrols during the summer, is part of a 20-strong team from Sihui that covers nine subway stations.

Combined, the stations have average daily passenger flow of over 2.1 million.

He said he has become adept at identifying a potential molester in a crowd.

"They act strangely," he said. "They will wander the platforms looking for 'targets', compared with most commuters, who just get on or off the train as quickly as possible."

Liu said they have found suspects exposing themselves, taking up-skirt photos and making deliberate physical contact with victims.

While spotting potential offenders has become easier over time, collecting evidence has not.

Video footage is the most powerful and direct proof. Liu said the undercover officers operate in teams of three and will record suspects from multiple angles, and if there is physical evidence, such as bodily fluids, they will detain the suspect immediately.

On June 29, Liu saw a man squeeze between two women in skirts as they boarded a packed train at Communication University of China Station at 8:45 am. Once in the car, the suspect put one hand between the women's hips, pretending it was accidental, and placed the other on his crotch.

Liu and his colleagues filmed the behavior on their smartphones. When the train reached Sihui Station about 15 minutes later, the man was taken into custody and the women agreed to give statements, resulting in the man receiving time in a detention house.

"Being able to detain an offender within just 15 minutes is not the norm, though," he said. "Typically, it can take several officers a lot of time and energy to nail down the proof."

In an online survey last year of 2,023 people－55 percent of whom were female－by China Youth Daily, 53 percent said they or their female friends had experienced sexual harassment on the subway.

Huang Qiongyu said a man had secretly shot video of her on a Beijing subway in May, but she didn't notice. A nearby passenger told her after the man exited the subway.

"I felt irritated and speechless," she said. "The people around didn't do anything to inform me. But they weren't to blame, because it wasn't their business."

Xu Weina, another Beijing commuter, said a man once touched her several times on the subway but pretended it was unintentional. "Everyone around me was staring at their phones, so no one noticed," she recalled. "I felt scared and helpless. I had no idea what to do other than get off at the next stop."

The survey also found that the first things people said they would do facing such things are ask security personnel for help (63 percent), shout to get attention (59 percent), and take photo evidence and report to police (54 percent). Six percent said they would remain silent.

However, Liu said that in reality, many female victims remain silent or are unwilling to cooperate with investigators, making the investigation more difficult.

"Some feel embarrassed or disgraced to speak about it, and some are in a hurry to get to work. We have to do a lot to persuade them. It's frustrating to release suspects for lack of evidence," he said.

In June last year, Chu Yan, a female police officer with Liu's team, suspected a woman was being harassed by a male passenger on the subway. To avoid embarrassing her, she typed "I'm police. Are you being molested?" on her phone and showed it to the woman, who nodded and typed her number into Chu's phone.

Chu detained the suspect, but when she called the woman several times to get a statement, the victim refused and even warned that if Chu kept calling, she would file a complaint. The suspect was released within 24 hours due to a lack of evidence.

A month later, the same man was detained again and the victim gave a statement. After that, Chu messaged the earlier victim the result, and she replied, saying she was sorry and expressing her gratitude.

Chu, who oversees suspect interrogations and victim interviews for the Sihui squad, said women tend to show less concern and talk more directly to her than to her male colleagues.

Also, "when suspects are not admitting to any wrongdoing, I will ask them to think about what if it happened to their wife or female relatives, and they usually become emotional", she added. "That usually leads to a confession."

Liu said the campaign, which is intensified during the summer and ramped down in winter months, has achieved good results and has helped reduce incidences of sexual harassment on subways. Most of the suspects who are punished realize their problems after education, and there are few repeat offenders, he said.

However, as Beijing's subway network continues to expand, and to ensure sufficient resources, police are building a cooperative mechanism with subway security guards on platforms and in cars who can report suspicious activity, including sexual harassment.

In June last year, Beijing deployed about 1,100 security guards on two subway lines as a pilot program. They have a security personnel permit issued by public security bureaus.

Each guard is assigned to watch over two subway cars, to deal with uncivilized behavior such as vagrancy and fighting, and to assist in emergency situations that require evacuation.

In June, Beijing deployed an additional 2,000. The security guards have not yet covered the whole subway network.

Mu Haitao, deputy head of security for Beijing MTR Co, which operates lines 4, 14 and 16 as well as the Daxing Line, said the guards are not police officers and do not have power of law enforcement, but they will offer help when needed.

"If passengers keep silent, the guards will not notice such cases in the packed cars. If they ask for help, the guards will help stop the suspect and report to police for arrest in a timely manner," he said.

Liu advised that at peak times, women should try to stand close to other female passengers. If they are harassed or molested, they should try to protect themselves first. If they are not sure whether the suspect is doing something inappropriate, they can give a glare and walk away, because, Liu said, the perverts are usually timid.

Females should avoid serious conflict with suspects and ask other passengers or guards for help, he said.