Construction began Friday to extend a metro line in Beijing built for the 2008 Olympics.

According to the municipal transport authorities, the No. 8 line will be extended both to the north and south. The added northern section will traverse 10.7 km from the south gate of the Beijing Olympic Park to the Huilongguan apartment community, outside the city's northern Fifth Ring Road.

The southern section will run 6.8 km from Beitucheng, near the northern Fourth Ring Road to the National Art Museum of China, authorities said.

A total of 12 stations will be added to the south-to-north metro line.

The construction of the northern section is expected to be completed in June, 2011 and the trial operation is scheduled for one year later, while the southern section will begin construction in June, 2012, they said.

Beijing now has nine subway lines in use, which stretch for 228 km. In 2015, they are expected to carry more than 10 million passengers a day with a total length of 561 km.