In endorsing the first "one person, one vote" direct election for a Chinese city to choose its leader, Beijing is billing the announcement as a milestone for political reform.

But democracy advocates say the restrictive framework falls well below international standards of universal suffrage, and will make it even harder for those with dissenting political views to make it onto the election ballot.

Under the framework endorsed by the National People's Congress, candidates put forward for election will require the approval of more than half Hong Kong's 1200-strong nomination committee, widely seen to be stacked with business elites and Beijing loyalists.

The nomination committee will put forward "two or three" candidates to a public vote, with the winner to be appointed chief executive by the central government.

Security was tightened in Hong Kong on Sunday evening ahead of a planned demonstration by pro-democracy movement Occupy Central outside the chief executive Leung Chun-ying's offices. More than 7000 police officers were expected to be deployed, the South China Morning Post reported.

Occupy Central's leaders had previously said it would stage a mass sit-in in the coming weeks to blockade the city's financial district, while thousands of university students had also pledged to strike from classes, if Beijing rejected its call for the public nomination of candidates.

Chen Zuoer, former deputy director of the Hong Kong and Macau Affairs Office, said that the movement was being "manipulated by Western countries to overthrow a regime", and that Beijing "would not tolerate such action".

But Occupy Central founder Benny Tai Yiu-ting emphasised that his campaign was not meant to challenge Beijing's sovereignty. "We are just seeking a fair electoral system, so that everyone can exercise their political rights," he said.

"Beijing is trying to present as 'democratic' a process over which it will retain complete control," Sophie Richardson, China director of Human Rights Watch, said. "I shouldn't be surprised if this prompts real outrage in Hong Kong, and it shouldn't compound that mistake by preventing people from expressing their views peacefully."

Wang Zhenmin, a prominent legal scholar and Chinese government adviser, who was flown to Hong Kong by the Chinese Ministry of Foreign Affairs to talk about the 2017 election, said it was time for "practical and realistic steps".

"Less perfect universal suffrage is better than no universal suffrage. Leave some room for future growth," he said.

with agencies