Chief Executive John Lee has warned that Hong Kong must remain on guard against attempts to endanger national security or disrupt public order.

Speaking to reporters ahead of the weekly Executive Council meeting, Lee said that a landmark subversion ruling handed down by a local court last week demonstrated that national security threats are very real.

Fourteen people were found guilty of conspiring to commit subversion by taking part in a plot to seriously obstruct, disrupt or undermine the duties and functions of the Hong Kong government. Another 31 people had earlier pleaded guilty.

Asked how the authorities would deal with people marking June 4 in public, Lee said all activities must be conducted according to the law.

"We should not forget the pain that we all went through in the attempted colour revolution which took place in 2019," Lee said.

Acts which endanger national security can happen "all of a sudden", and perpetrators may use different excuses to disguise their intentions, he warned.

Law enforcement agencies will take action according to existing provisions, Lee stressed, including the Public Order Ordinance and the Safeguarding National Security Ordinance.