Beijing police have set up special teams at several public security stations to fight sexual harassment on subways as summer usually sees a rising number of such cases in public transportation.

The Sihui police station has launched a special team of 30 officers to investigate sexual harassment cases inside subway sections within its jurisdiction, passing through nine subway stations, including Guomao and Sihui, with an average daily passenger flow of over 2.1 million, according to a police news release on Friday.

The area under the jurisdiction of the Sihui police station includes a central business district and some universities, with many young female passengers.

The station has detained 23 subway molesters this year, and as summer comes, molestation cases are more likely to happen, said Wang Yongchao, deputy head of the police station.

However, punishing molesters is no easy job as it's hard to obtain evidence, according to the police. Wang said videos are the most powerful and direct evidence in such cases, but it is difficult to shoot videos because it may alert the suspects. The congestion inside the subway also adds to the difficulty of shooting videos, he said.

"It sometimes takes the police several weeks to find enough evidence to detain a suspect, Wang said.

Many female victims choose to remain silent or are unwilling to cooperate with the police after they are violated, which also makes the investigation more difficult, he said.

"Some may feel embarrassed or disgraced to speak about it, and we have to do a lot of work to persuade them," Wang said.