Hong Kong's pro-democracy activist Joshua Wong has been freed from prison and pledged to join the ongoing mass protest movement demanding the city's Beijing-backed leader, Carrie Lam, steps down over a controversial extradition bill.

Wong, 22, was released on Monday from the Lai Chi Kok correctional facility after completing a two-month sentence for his part in the so-called Umbrella Movement that brought the city to a standstill for more than two months in late 2014.

The move comes as a political crisis in the Chinese-ruled city enters its second week amid growing uncertainty over the fate of Lam and the extradition bill, which she suspended "indefinitely" on Saturday.

Before being jailed, Wong had called for the Hong Kong government to scrap the legislation, under which it would be allowed to extradite people to any jurisdiction in the world with which it currently has no existing formal agreement - including mainland China.