He made the comment following the death of a 50-year-old man who stabbed himself after allegedly knifing a police officer in the back in Causeway Bay on Thursday’s handover anniversary.
Speaking to reporters, Tang said there were people trying to portray the suspect as a hero or even calling him a martyr online over the past two days.
The security chief said he found it chilling that a parent took their child to mourn the attacker.
“Do [they] want our next generation to all become bloodthirsty terrorists?” he asked.
Tang pointed out the public should clearly see that there are “evil forces” trying to incite hatred and nurture terrorism in Hong Kong.
He said authorities will work together to fight terrorism and those who promote it.
Tang added any act that is illegal in the real world is also illegal on the internet.
“When you promote violence and ask people to come out to break the law, you are breaking the law. When you promote acts that endanger national security, you are breaking the law. Our police force and other law enforcement agencies will try our best to pluck you out and send you to prison.”
Meanwhile, Chief Secretary John Lee warned against what he called local terrorism.
In his first blog post as the city’s second ranking official, Lee said that while the national security law has brought Hong Kong’s chaos under control, those who are pro-independence and saboteurs have not given up.
He described the July 1 attack on the police officer as an attempted murder and a lone-wolf terrorist act.
The chief secretary stressed that endorsing the suspect and mourning or paying tribute to him could amount to calls for destroying or threatening Hong Kong’s safety and order.