Under the mini-constitution, the Basic Law, Hong Kong is guaranteed wide-ranging autonomy for “at least 50 years” after 1997 under a “one country, two systems” formula praised by Xi. It also specifies universal suffrage as an eventual goal.

Pro-democracy activists in Hong Kong were expecting tens of thousands of people to attend an annual protest rally on Sunday to mark the 21st anniversary of the city's return to Chinese rule as tensions simmer over Beijing's tightening grip.

Turnout will be a key indicator of public sentiment at a time when the opposition in Hong Kong has seen activists jailed and others barred from running in a by-election.

An organiser of the Sunday protest, Sammy Ip, said the rally would not target Hong Kong leader Carrie Lam but focus on a broad push against Beijing's perceived encroachment into the former British colony.

At a ceremony early on Sunday to mark the anniversary, Chief Executive Lam asserted that the “one, country, two systems” framework under which the financial hub is governed remains intact under her watch.

Ms. Lam took over as governor of the former British colony a year ago, pledging at a ceremony attended by Chinese President Xi Jinping to be accountable to both Beijing and Hong Kong.

“Without fear, we correctly deal with our relationship with the central government. And we promote a stronger understanding of the constitution, the Basic Law, and national security in all sectors,” Ms. Lam said at a Sunday morning cocktail reception.

Also present at the ceremony were the three former Chief Executives - Tung Chee-wah, Donald Tsang and Leung Chun-ying, as well as senior mainland officials.

Under the mini-constitution, the Basic Law, Hong Kong is guaranteed wide-ranging autonomy for “at least 50 years” after 1997 under a “one country, two systems” formula praised by Xi. It also specifies universal suffrage as an eventual goal.

Beijing's refusal to grant full democracy to Hong Kong triggered massive street protests in 2014 and deepened resentment toward China's perceived growing encroachment on the territory, where its influence in nearly every facet of life has increased.

While critics in Hong Kong have questioned Ms. Lam's commitment to the autonomy and freedoms under the “one country, two systems” formula, Beijing has praised her leadership.

“You have adopted a series of policies ... to promote Hong Kong's economic development and social harmony,” China's Vice Premier Han Zheng said after meeting Lam in Beijing on Tuesday.

“I congratulate you,” he said.

Ms. Lam was chosen by a largely pro-Beijing committee of some 1,200 people in the city of 7.3 million.

Her approval ratings have dipped since then. A University of Hong Kong survey of 1,000 people put her approval rating at 54.3 points, down from 61.1 points a year ago.