Hundreds of Hong Kong police forcibly removed kicking and screaming protesters from the central business district as they took part in a mass rally demanding greater democracy from Communist Party rulers in Beijing.
The pro-democracy march yesterday, which organisers said attracted more than 510,000 people, and a subsequent sit-in by mainly student groups could be the biggest challenge yet to China, which resumed control over the former British colony in 1997.
Many of the more than 1,000 protesters linked arms in a bid to resist efforts to remove them but they were taken away one at a time, in some cases by three or four police, as activists kicked, screamed and punched before being bundled on to buses.
"I have the right to protest. We don't need police permission," the crowd chanted as they sat sweltering in Hong Kong's summer heat and humidity.
Activists who refused to leave were taken in buses to the police training school in Hong Kong.
More than 500 people were arrested, with some charged with participating in an unauthorised assembly and obstructing police.
It was unclear how long they would be detained. About 50 were released without charge.
In one of the first moves of what is expected to be a hot political summer, the demonstrators were demanding greater democracy in elections for Hong Kong's leader, or chief executive, in 2017.
They want nominations to be open to everyone. China's leaders want to ensure only pro-Beijing candidates are on the ballot.
Hong Kong returned to China with wide-ranging autonomy under the formula of "one country, two systems", allowing protests such as yesterday's march to take place.
But China bristles at open dissent, especially over sensitive matters such as demands for universal suffrage and the annual 4 June vigil in Hong Kong to mark the crackdown on pro-democracy protesters in Beijing in 1989.
Such protests, even by one or two people, would be met by stern punishment elsewhere in China.
Occupy Central with Love and Peace, the group behind an unofficial referendum on democracy which drew nearly 800,000 votes, has threatened to lock down Central as part of its campaign.
"The voice of the Hong Kong people has been loud and so clear. If they (Beijing and the Hong Kong government) choose to ignore it, they will have to pay the price," said Helena Wong of the Democratic Party.
"Occupy Central is the last resort ... We will keep it as our last weapon if we do not have true democracy."
Hong Kong Chief Executive Leung Chun-ying said his government would do its "utmost" to move towards universal suffrage and stressed the need for stability. Beijing's Liaison Office in Hong Kong said China "firmly supported" universal suffrage for Hong Kong, and "its sincerity and determination is unswerving".
The overnight protest threatened to disrupt traffic as people returned to work following a public holiday on Tuesday to mark the 17th anniversary of Hong Kong's return to China.
Some buildings in Central, including HSBC's headquarters, were ringed by barriers, although these were largely cleared as business resumed.