China's President Xi Jinping said on Saturday, July 1 Hong Kong was freer than ever before but laid down an uncrossable "red line" for any challenge to Beijing's authority as the city marked 20 years since it was handed back by Britain, AFP reports.

Xi spoke in a televised address after swearing in new Hong Kong leader Carrie Lam as pro and anti-Beijing protesters clashed close to the ceremony.

Lam was selected by a pro-China committee, as were her predecessors, and is already being cast by critics as a China stooge in a city where many are angry at Beijing's tightening grip on the freedoms of its nearly eight million people.

A huge security operation shut down large parts of Hong Kong for Xi's three-day visit, reflecting Beijing's concern that there should be no embarrassment ahead of a key Communist Party congress later this year which is expected to cement his position as the most powerful Chinese leader in a generation.

The trip was his first since becoming president in 2013 and comes three years after mass pro-democracy rallies crippled parts of the city for months.

Xi said Saturday that any threat to China's sovereignty and security or to the power of the central government "crosses the red line and is absolutely impermissible".

He also warned against anyone endangering Hong Kong's constitution or using the city "to carry out infiltration and sabotage activities against the mainland".

The Sino-British Joint Declaration gave Hong Kong rights unseen on the mainland through a "one country, two systems" agreement, lasting 50 years.

There are growing fears that freedoms guaranteed by the handover deal are now under threat, with Chinese authorities accused of abductions and interfering in a range of areas, from politics to media and education.