Protesters are taken away by police officers after hundreds of protesters staged a peaceful sit-ins overnight on a street in the financial district in Hong Kong Wednesday, July 2, 2014, following a huge rally to show their support for democratic reform and oppose Beijing's desire to have the final say on candidates for the chief executive's job. Kin Cheung/Associated Press

HONG KONG—Hong Kong's pro-democracy forces got a shot in the arm from a day of marches and a night of protests in the city, raising the possibility of more demonstrations later this year.

Police arrested 511 people at an overnight sit-in in Hong Kong's Central business district Wednesday morning after thousands of protesters—organizers estimated 510,000, while police said the crowd hit 98,600 at its peak—marched across the city on Tuesday on the anniversary of Hong Kong's handover to China.

Participants in the sit-in on Central's Chater Road, organized by the pro-democracy Hong Kong Federation of Students, said that despite the arrests they felt encouraged by the demonstration, which they deemed a success.

"We are more hopeful now," said Jack Lee, a Hong Kong University student who participated in the Chater Road protest but wasn't arrested. "We have sent a clear message to all Hong Kong people, that civil disobedience can be peaceful and rational."

"It's certain that we can win more support from this," Mr. Lee said.

Hong Kong will see a 'rough period' in politics over the next six months and possibly a new mass street protest, says Danny Gittings of the University of Hong Kong. The WSJ's Ramy Inocencio recaps July 1st protests marking the city's return to China.

A spokesman from Chief Executive Leung Chun-ying's office declined to comment on the protests, but Secretary for Security Lai Tung Kwok said the police handled the cleanup professionally.

The protests center on whether Hong Kong voters will be given a mechanism for nominating candidates for chief executive, the city's top post, in the 2017 elections. While the government has committed to realizing what it calls universal suffrage in chief executive elections by that year, it is unclear whether that reform will include a robustly democratic nominating process.

The chief executive was last chosen in 2012 by a 1,200-member committee composed largely of pro-Beijing organizations and business groups. A pro-democracy coalition, Occupy Central, has called for protesters to paralyze the city's financial district later this year if a promised government blueprint for universal suffrage fails to meet their standards.

At stake, besides Hong Kong's electoral process, is the city's relations with Beijing, which takes a dim view of democratic advocacy. A front-page editorial in Communist Party mouthpiece People's Daily on Wednesday said that self-governance in Hong Kong "has always had clear boundaries and standards, namely that Hong Kong should be mainly governed by Hong Kongers who love their country."

Citing former paramount leader Deng Xiaoping, the editorial defined patriotic Hong Kongers as people who "respect their nation, sincerely support the motherland's recovery of sovereignty over Hong Kong, and do not damage Hong Kong's prosperity and stability."

Despite concerns that the protests could rattle Hong Kong investors, the Hang Seng Index rallied 1.6% to 23549.62 during the day to hit a high for the year as traders reacted to better-than-expected manufacturing data across much of emerging Asia, released during Tuesday's holiday.

Demonstrators gathered for a pro-democracy rally in Hong Kong, where demands for universal suffrage—promised by Beijing to begin in 2017—have grown, but the city's government has wrestled with the question of how to introduce direct elections. Agence France-Presse/Getty Images

Tens of thousands of protestors gather in Hong Kong Tuesday to protest the Chinese government and demand more democracy. This comes days after 800,000 residents voted in a mock referendum aimed at rallying support for a full democracy. Photo: AP

In addition to the Chater Road sit-in, a separate overnight protest was held in front of the offices of the chief executive by a different student group called Scholarism. The protest ended around 9 a.m. Wednesday without any arrests, that organization said.

Max Lau, an 18-year-old spokesman for Scholarism, said the sit-in was a "mature student movement," defying the perception that students are impulsive or irrational. "Instead, we are willing to bear the legal consequences of civil disobedience in this action and we did it peacefully and orderly," Mr. Lau said.

"We expect further escalation of our movement such as more protests to encircle the chief executive's offices in coming months," said Mr. Lau.

The showdown on Chater Road was foreshadowed early Wednesday by the police, who said they would start to clear the protesters from the area before dawn. Organizers called for protesters to remain on the road until 8 a.m. By 3:30 a.m., police were slowly carrying individual protesters off the street and loading them onto waiting buses. Some of the protesters shouted and struggled as police carried them off, while others walked or were carried silently. Wheelchairs were also used to move the protesters. Police held signs over the crowd telling them to board the buses.

Protestors in Wan Chai. Darren Hayward/The Wall Street Journal

A crowd of onlookers outside the police cordon cheered on the protesters, leading a spirited countdown to 8 a.m., at which point the remaining few stood up to applause, flashing victory signs. While the crowd called for their release, they were escorted one by one to a bus. By 9 a.m., most of the crowd had dispersed and the area around Chater Road gave way to its morning rush-hour routine, with suited professionals walking to work under bright sunshine.

Protesters were arrested on suspicion of "unauthorized assembly and obstructing police officers," according to a Hong Kong police spokeswoman, who declined to comment on whether such a large number of arrests was unusual for a protest. A statement released on the police force's website said the force expressed regret "over the uncooperative and illegal actions of the participants" in the Chater Road protest.

The protesters have yet to be formally charged. Protest organizers said that that the arrested demonstrators were taken by bus to a police college in Wong Chuk Hang, on the south side of Hong Kong Island, where organizers called for more protesters to gather Wednesday afternoon. The police spokeswoman declined to comment on where the protesters were taken.

Organizers say this may not be the last instance of civil disobedience in Hong Kong this year. "The stronger the oppression from the authorities, the stronger the rebound from the public," said Joshua Wong, a leader of the Scholarism group.

—Gregor Stuart Hunter contributed to this article.

Write to Ned Levin at, Chester Yung at and Edward Ngai at