Pro-democracy demonstrators in Hong Kong blocked a second major thoroughfare, this time across the harbour, after police had earlier used tear gas to disperse huge crowds in the city's centre.

Around three thousand demonstrators blocked a major road in the city's congested Mongkok district in Kowloon, bringing traffic to a standstill and opening up a second significant flash point in the key financial hub.

A dozen police looked on helplessly as the demonstrators took over Nathan Road, chanting slogans and wrapping their eyes in clingfilm to protect themselves in the event of pepper spray being deployed, an AFP reporter said.

The move risked stretching police resources even thinner as riot officers used repeated volleys of tear gas to try and disperse tens of thousands of demonstrators who gathered at a major road on the island containing the city's financial district.

Some of those demonstrating in Mongkok said they took their protest across the harbour because it was safer there.

"We need to strike for freedom and for our democracy. We've come to Mongkok because it's very dangerous now in Central and Admiralty," 20-year-old student Calvin Chan told AFP, referring to the two districts where tear gas had been deployed.

Pro-democracy groups are locked in a struggle with the city's authorities over China's refusal to grant unfettered democracy.

Last month Beijing said Hong Kongers could elect their next leader in 2017 but only those who had been vetted would be allowed to stand -- a decision branded a "fake democracy" by protesters.