Chief Executive John Lee on Tuesday said the 20-year jail term given to Jimmy Lai is a "solemn warning" to those attempting to collude with foreign forces to endanger national security.

The 78-year-old Apple Daily founder was imprisoned on two counts of conspiracy to collude with foreign forces and a third charge of conspiracy to print seditious material.

Lai's sentence is "a heavy punishment that sends a solemn warning against malicious plots of collusion with external elements", the chief executive said, adding that the court's ruling manifested that the rule of law is upheld.

"His conviction is supported by overwhelming evidence and he deserves the punishment after all the harm that he has done," Lee said.

The city's leader also criticised Western countries and politicians for "badmouthing" Hong Kong's efforts in safeguarding national security.

"Some of the people we are taking action against are their pawns, are their representatives, are under their influence, to endanger Hong Kong's and our country's national security. It will be against their interests that we successfully neutralise their attempts," Lee said.

Asked if the authorities will appeal the sentence, Lee said the Department of Justice and police will study the court's decision in detail.