(Photo Credit: Kin Cheung/AP Photo)

ABC News' Xiao Kaojing reports:

HONG KONG - Tens of thousands of people gathered in Central Hong Kong today to join a pro-democracy march - expected to be the largest such protest in the country's history - calling for open voting in the 2017 chief executive elections amid worries about Beijing's increased heavy-handedness.

After the official flag-raising ceremony this morning marking the 17th anniversary of the handover of Hong Kong from the United Kingdom to China, the region's chief executive, Leung Chun-ying, warned protestors against doing anything to damage the territory's "prosperity and stability."

But Leung offered no detailed plans for the 2017 election that guarantees voters a genuine choice among candidates.

Crowds Gather in Hong Kong to Mark 25th Anniversary of Tiananmen Massacre

People had already started to gather this morning at Victoria Park for a rally that had been scheduled for 3 p.m., which would march from there to the city's central business district. Protestors burned copies of portraits of Leung after he failed to deliver a blueprint for the 2017 election.

The anniversary of the handover has become an annual day of protest in Hong Kong. Officials moved this year's event to a new stage after Beijing released a White Paper on "one country, two systems" June 10, which emphasized its total authority over Hong Kong.

The paper sparked fears that Hong Kong's high degree of autonomy and independent judiciary could be under threat. Many residents worry that Beijing has been encroaching on their civil liberties, free press and independent judiciary.

The protestors held banners and slogans proclaiming, "We want public nomination" and "We do not fear the White Paper." Two protesters carried a coffin to remember the victims of the June 4, 1989, Tiananmen Square crackdown, the South China Morning Post reported.

Ten activists from the pro-independence group Hongkongers rallied Sunday outside the British Consulate-General in Admiralty, urging London to take back Hong Kong. According to the South China Morning Post, one activist, Billy Chiu Hin-Chuang, said: "Hong Kong should be made an independent state. Hong Kong is our country. We are not Chinese."

Two Hong Kong student groups - the Federation of Students and the student-led group Scholarism - planned an overnight occupation at the central business district. At least 1,000 students had planned to stage a sit-in after the main pro-democracy rally in Chater Road and outside the chief executive's office in Admiralty until 8 a.m. Wednesday morning.

A Hong Kong police spokesman said police traffic-management officials prepared for about 150,000 people at the rally, adding that they had sufficient manpower to handle the crowds.

Scholarism organizer Joshua Wong Chi-fung said members were prepared to be arrested but would avoid clashes with the police to keep protests peaceful.