Chinese President Xi Jinping swore in Hong Kong's new leader Carrie Lam with a stark warning that Beijing will not tolerate any challenge to its authority in the divided city as it marked the 20th anniversary of its return from Britain to China, BBC reported.

But clashes have taken place between pro-democracy and pro-Beijing demonstrators close to the site, with several arrests made. 

The pro-democracy party, Demosisto, said police had arrested five of its members, and four members from the League of Social Democrats. 
During the protest actions demonstrators were chanting "end one-party dictatorship".

The UK returned Hong Kong to Chinese rule in 1997 under a "one country, two systems" formula which guarantees wide-ranging autonomy and judicial independence not seen in mainland China.