There will be "no leniency or mercy", the spokesman added.
"Protesters have been frequently using extremely unsafe tools to attack the police in recent days, constituting serious crimes with sprouts of terrorism emerging".
Video footage obtained by the Hong Kong Free Press appears to show not only uniformed police making arrests, but officers dressed in the black t-shirts and hard hats that have become a symbol of the resistance.
Earlier in the day, police declined to say if they would move to clear the demonstrators.
Assistant Police Commissioner Mak Chin-ho, who also faced a volley of questions about Sunday's violent scenes, said there was no evidence to suggest a woman had been shot by a police projectile.
One female protester was seen being treated by paramedics, after she was hit in the face by a beanbag round.
A train station in Kwai Fong filled with smoke after about a dozen police officers fired tear gas inside.
Tang said that on August 11, some people turned a deaf ear to the police's objection and got involved in unauthorized rallies. "There is a high risk of violent confrontation between protesters and police, or criminally-linked individuals".
Hong Kong's Airport Authority cancelled all departing flights not yet checked in on Monday afternoon as anti-government protests continue in the airport.
Police are now attempting to remove protesters from the airport and there are long queues for any form of transport away from the airport.
One passenger, who wishes to remain anonymous, told the BBC they had been left stranded at the airport with "no food and no drinking water".
News of tear gas and arrests outside the airport began to filter in and news reports were advising that all transport had been shut down and the airport area was to be avoided. "People are sleeping on conveyor belts and hard floors".
Tear gas was sacked at the blackshirted crowds in districts on Hong Kong island, Kowloon and the New Territories, with one young female medic hospitalized after being shot in the right eye, triggering a protest by medical workers who wore bloodied patches over their eyes.
"So, the demonstrators in Hong Kong, I think, don't see the bigger picture, they don't know what America's really like, and they are being taken for a ride".
Hong Kong has experienced more than two months of mass protests calling for democratic reforms and an independent inquiry into police conduct in the wake of proposed legislation that would have sent people accused of a crime to mainland China for trial.
Although the government has now suspended the bill, demonstrators want it to be fully withdrawn.
The latest escalation follows a weekend of bloody clashes between the police and protesters.
Cathay Pacific is based in Hong Kong, which though a part of China is governed under a separate legal framework granting the city certain political and legal freedoms not available on the mainland. It is run under a "one country, two systems" agreement that guarantees it a level of autonomy.