China's President Xi Jinping says Hong Kong is freer than ever before but has laid down an uncrossable "red line" for any challenge to Beijing's authority as the city marks 20 years since it was handed back by Britain.
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 has 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 is his first since becoming leader 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 warning comes after the emergence of young activists calling for self-determination or even full independence for Hong Kong, which has infuriated Beijing.
Xi insisted that Hong Kong had "more extensive democratic rights and freedoms than at any other time in its history" and pledged to uphold its semi-autonomous status.
But Beijing's foreign ministry declared on Friday that the document signed by Britain and China which initiated the handover "is no longer relevant".
The Sino-British Joint Declaration gave Hong Kong rights unseen on the mainland through a "one country, two systems" agreement, lasting 50 years.
Protesters targeted
There are growing fears that those freedoms are now under threat from an assertive Beijing, with Chinese authorities accused of abductions and interfering in a range of areas, from politics to media and education.
Xi called on Hong Kong to "improve" its systems in order to uphold Chinese sovereignty and to "enhance" education to promote China's culture and history.
A proposal to include patriotic education in Hong Kong schools met with huge protests in 2012 and has since been shelved.
Pro-China protesters targeted a small march by activists in memory of the victims of Beijing's 1989 Tiananmen Square crackdown on Saturday morning as officials gathered for the swearing in.
As the pro-democracy campaigners prepared to carry a makeshift coffin towards the convention centre, as they do each year, a man ran across the street and kicked it.
Flag-waving pro-China protesters then blocked the march as police struggled to separate the two sides.
Democracy campaigners were taken away in police vans and released soon after.
Activists say they have been repeatedly followed and attacked since Xi flew in on Thursday.
Lam's swearing in by Xi is deeply symbolic for frustrated activists who pushed for fully free leadership elections during the 2014 Umbrella Movement rallies but failed to win concessions.
Those protests were sparked by a Beijing-backed political reform package which said Hong Kong could have a public vote for leader, but that candidates must be vetted first.
The proposal was voted down in parliament by pro-democracy lawmakers and the reform process has now stalled.
Lam has made no commitment to revisit it soon.
