Construction on the latest expansion of Beijing Subway Line 1, which became China's first urban subway line when it opened in 1969, began on Thursday, local authorities said.

The expansion will extend approximately 21 kilometers into Beijing's southwestern suburbs, and nine stations will be added to improve the city's urban rail transit network, according to the Beijing Major Projects Construction Headquarters Office.

Line 1 of the Beijing Subway was designed and constructed independently by China, and was put into operation in October 1969. The line has undergone several expansions in recent years, linking the Chinese capital's core area and its sub-center, the headquarters said.

The latest expansion will also help relieve Beijing of functions non-essential to its role as the national capital, and facilitate regional coordinated development.

Beijing's total urban rail transit mileage totaled 836 kilometers by December 2023, ranking first in China.