Schools, now on Lunar New Year holidays, would remain shut until Feb 17, while inbound and outbound flights and high speed rail trips between Hong Kong and Wuhan would be halted.
Lam said all official visits to the mainland and official Lunar New Year celebrations would be scrapped immediately.
She also said she had sought the assistance from China's State Council to ensure mask supplies were adequate.
The city's health authorities have confirmed 5 coronavirus cases, all linked to Wuhan, where the virus first appeared, with a further 122 people being treated as suspected of having the disease.