The July 1 protest march ended at Chater Road on Tuesday night, but many demonstrators went on to take part in an overnight sit-in, organised by the Federation of Students and activist group Scholarism.
The protesters were eventually cleared from Chater Road by police at around 8am yesterday morning. Student leaders described the sit-in as a rehearsal for Occupy Central.
Police announced that 511 people had been arrested on charges of illegal assembly and obstructing police officers. Most protesters put up little resistance, but continued chanting political slogans as they were being taken away.
Agnes Chow Ting, a Scholarism activist, was unhappy with the way the police acted. "Police were trying to prevent the demonstrators from gaining access to the protest area by erecting steel barricades," she said. "It reveals the unfairness of the procedures for public safety."
The arrested protesters were taken to the Hong Kong Police College in Wong Chuk Hang. As of yesterday lunchtime, fewer than 50 of those arrested had been released, according to Joshua Wong Chi-fung, co-founder of Scholarism.
The student leader's Facebook post said there is only one telephone line available for the protesters to call for legal assistance.
"There are even rumours that police didn't allow protesters to order breakfast," he added.
Organisers said 510,000 Hongkongers had taken part in Tuesday's march, making it the biggest since 2003. However, police said at its peak, just 98,600 were marching, while the University of Hong Kong's public opinion programme estimated the turnout at between 154,000 and 172,000.
Staff writer, with additional reporting from YP cadet Holiday Chan