BEIJING, Jan. 16 -- Highways were closed Thursday morning as smog blanketed Beijing with air quality readings reaching the most polluted level.

"The skyscrapers in the CBD have all vanished. I could barely see anything from the skywalk where I transfer subway lines every morning," a citizen surnamed Li said.

The smog started Wednesday afternoon and pollution levels continued to rise during the night, according to Beijing Municipal Environment Protection Department.

Beijing's average PM 2.5 index, which measures airborne particles smaller than 2.5 micrometers, stood at 400 to 500 micrograms per cubic meter in most of the city at 6 a.m. on Thursday.

The municipal government issued a yellow smog alert at 7 a.m., as visibility in some parts of the city is expected to be less than 500 meters. Yellow is the second-lowest level in China's four-tier color-coded weather alert system.

Four highways linking Beijing to Shanghai, Daqing to Guangzhou, Beijing to Harbin, and Beijing to Pinggu were temporarily closed due to the smog.

The smog is forecast to last until Friday morning.