Chinese President Xi Jinping has warned against “impermissible” challenges to Beijing’s authority over Hong Kong.
Mr Xi was speaking at the swearing-in of the territory’s new leader Carrie Lam, as it marks 20 years since its handover to China from Britain.
He took part in a series of lavish events, including a flag-raising ceremony, amid tight police security.
But clashes took place between pro-democracy and pro-Beijing demonstrators close to the site. Several were held.
The visit was Mr Xi’s first to the city since becoming leader in 2013 and comes three years after mass pro-democracy rallies crippled parts of the territory for months.
He oversaw the swearing in of Ms Lam, the newly-elected chief executive of the territory, along with the rest of her cabinet. She is Hong Kong’s first female leader.
In a speech he said that Hong Kong needed to “improve its systems to uphold national sovereignty, security and development interests”.
“Any attempt to endanger China’s sovereignty and security, challenge the power of the central government… or use Hong Kong to carry out infiltration and sabotage activities against the mainland is an act that crosses the red line and is absolutely impermissible,” he said.
He added that Hong Kong now enjoyed more freedom than ever before.
But while Hong Kong’s Basic Law guarantees wide-ranging freedoms under the “one country, two systems” formula, Beijing’s refusal to grant universal suffrage has triggered sometimes violent unrest.
In Saturday’s protests, pro-democracy party Demosisto said police had arrested five of its members, and four members from the League of Social Democrats.
Among those said by the group to have been arrested was Joshua Wong, the leader of the so-called umbrella protest movement.
