Beijing is tightening security checks in subway stations during the annual two sessions of the nation's top legislature and political advisory body held in the capital.

Six key stations on Line 1 have conducted compulsory security checks of subway passengers and their belongings, Beijing Mass Transit Railway Operation Corp said in a statement on Monday.

Of the six stations, Tian'anmen East and Tian'anmen West initiated a pilot program in December to check passengers. The other four stations were Xidan, Wangfujing, Dongdan and Jianguomen.

Metal detectors have been set up in those stations to scan passengers before they board trains, much like passing an airport checkpoint.

Passengers currently undergo security checks only for their belongings before boarding a subway. The enhanced security checks in the six stations have slowed passenger flows during rush hours because of limited resources, according to the operator, which runs 14 subway lines in the city.

Its Beijing operation includes 235 stations and 517 security checkpoints.

From March 3 to 7, the 14 lines handled an average of 8.91 million trips each day, according to the operator.

As large crowds must pass the security checks, passengers were asked to carry less.

Jia Peng, a subway company spokesman, said that after the annual two sessions, the six stations on Line 1 will continue the security checks using the metal detectors.

Checks "will gradually be applied in other stations" once approved by the authorities, he said.