President Xi Jinping said Hong Kong was freer than ever before but warned against "impermissible" challenges to Beijing's authority over the city in a speech Saturday to mark 20 years since it was handed back to China by Britain.

"Any efforts to endanger national sovereignty, challenge the authority of the central government and the Hong Kong Basic Law (the city's constitution), and use Hong Kong to penetrate and sabotage the mainland, cross the bottom line and are absolutely impermissible," he said in a televised address after swearing in new city leader Carrie Lam.