There were chaotic scenes on Sunday, with protesters screaming "Shame!" at police as they tried to shield themselves from the clouds of gas, AFP reporters said.
Several scuffles broke out between police in riot gear and demonstrators angered by the use of tear gas, which is rare in Hong Kong. An elderly woman was seen being carried away by protesters.
Police had earlier used hand-held pepper spray on demonstrators who had spilt onto a major multi-lane highway after breaking through barricades set up to stop people swelling the crowds camped outside Hong Kong's government headquarters since Friday.
Traffic had ground to a halt on busy Connaught Road, with police forced to retreat as the protesters rushed towards the crowds outside government headquarters on the other side. They cheered and embraced each other in the middle of the road, a major city artery usually filled with whizzing taxis and buses.
China, which stations a military garrison in Hong Kong, said it was confident the city's administration could handle the protest.
The extraordinary scenes came at the climax of a week of student-led action against China's refusal to grant full democracy to the former British colony.
Beijing said that while it would allow elections for Hong Kong's leader in 2017, it would insist on vetting the candidates.
Prominent pro-democracy group Occupy Central threw its weight behind the protests on Sunday, saying they were bringing forward a mass civil disobedience campaign that had been due to start on October 1.
The group had sparked months of heated debate in the city of seven million over its plan to bring Hong Kong's financial district to a standstill with a mass sit-in. On Sunday they appeared to have come close to reaching that goal.
Hong Kong's leader, Chief Executive Leung Chun-ying [branded Occupy Central's] activities illegal as they were designed to paralyse the city.
Hong Kong's leader, Chief Executive Leung Chun-ying, told a press conference his administration was "resolute in opposing the unlawful occupation actions by Occupy Central", branding its activities illegal as they were designed to paralyse the city.
He said his government would hold more public consultations on the planned political changes - a move already scheduled before the protests.
Traffic on the busy Connaught Road had already been heavily slowed by a man who had climbed onto the outside of a bridge and threatened to jump unless protesters were allowed through police lines. He surrendered shortly after the protesters broke through.
The crowds of protesters camped outside the city's government headquarters had swelled on Saturday night to more than 10,000, with scuffles with police overnight as lines of officers pushed back surges of people with riot shields.
