Policemen look on as some of their colleagues wait on coaches to transport protesters in the Central district after a "pro-democracy" rally in Hong Kong on Wednesday. Photo: AFP

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All 511 protesters detained by police during a sit-in in Hong Kong's Central commercial district earlier Wednesday were released before midnight.

Police gave warnings to 486 protesters, allowing the remaining 25 to leave on bail, according to Radio Television Hong Kong.

Those allowed to leave on bail must report back to police in late July or early August, police said. Among them were three men who were found with weapons such as knives on their persons. A 21-year-old woman was detained for "assaulting the police."

The protesters, including members of Hong Kong Federation of Students and some members of the region's pan-democrat parties, began the gathering at around 8 pm on Tuesday.

Police moved in on the gathering at around 3 am, taking away the protesters one by one.

A police statement expressed the force's regret at protestors' uncooperative attitude and breaches of the territory's laws, and emphasized that its policemen have been professional, showing restraint and using "proper and necessary" force when having no other choice.

Tai Yiu-ting, an organizer of the Occupy Central with Love and Peace, said that, although he did not agree with the timing of the protest, his group would offer legal help for the arrested.

In the meantime, members of another student group, Scholarism, continued a vigil begun outside chief executive CY Leung's office building at 10 pm on Tuesday, but did not manage to meet with Leung as planned.

"We will remain non-violent and try to avoid coming into physical contact with police," said Wong Chi-fung, one of Scholarism's organizers.

Organizers of Tuesday's rally claimed that 510,000 people took part, while police estimated that only 98,000 people had participated. The Hong Kong University Public Opinion program said the number fell between 154,000 and 172,000.

"The turnout of the rally does not represent the whole of Hong Kong," Chang Chak-yan, Chinese University political scientist and co-founder of Silent Majority told the Global Times.

"They are only representative of the existing supporters of the pro-democracy camp, who are trying to mobilize support so as to make their way into the pool of candidates for 2017 elections for chief executives," he said.

He added that the overnight sit-in was intended to be a rehearsal to test the waters before Occupy Central officially commence, adding that protestors wanted to see how the government would react .

Separately, China on Wednesday urged the US to stop commenting on Hong Kong's internal affairs. Foreign Ministry spokesman Hong Lei said Hong Kong people's basic rights and freedom have been fully guaranteed and the universal suffrage in 2017 must comply with the Basic Law.