HONG KONG, Aug 18 (Reuters) - Hong Kong police said on Wednesday four students were arrested for "advocating terrorism" after their student union passed a motion last month mourning the death of a 50-year-old who stabbed a policeman before killing himself. Police have said an officer was stabbed from behind on July 1, while on duty with other policemen preventing protest gatherings on the anniversary of the former British colony"s return to Chinese rule in 1997. The man then stabbed himself in the chest with the knife and died later in the hospital. The policeman, 28, suffered a punctured lung, but survived what Secretary for Security Chris Tang described as a terrorist act by a "lone wolf." read more Shortly after the attack, a few dozen members of the Hong Kong University student union passed a motion, since withdrawn, to commemorate the 50-year-old"s death and "appreciate" his "sacrifice." The union leaders have resigned and apologised for the "inappropriate" motion. The union"s campus office has since been raided by national security police and the university has severed ties with the union and banned about 30 students who signed the motion from entering its premises. "The motion is very shocking," Senior Superintendent Steve Li told reporters. "It tried to rationalise and glorify terrorism," Li said, adding that it "encouraged people to attempt suicide" and it did not align with "our moral standards." Li said the four were aged 18-20 and police will interrogate the students who voted in favour of the motion. Hong Kong police does not name people under investigation. Hong Kong has been polarised since protesters took to the streets in 2019 demanding greater democracy and accountability for what activists called police violence, an accusation authorities have repeatedly rejected. The protests were ended by the coronavirus pandemic and a sweeping national security law imposed by Beijing last year. Since the law was introduced, the government"s most prominent opponents have been jailed or fled abroad. Critics say the legislation has crushed the city"s wide-ranging rights and freedoms, while supporters say it has restored stability. After the July 1 attack, some people went to the scene of the stabbing to lay flowers, drawing condemnation from authorities, including city leader Carrie Lam. Lam urged parents, teachers and others at the time to observe the behaviour of teenagers and report those who break the law to the authorities. read more Writing by Marius Zaharia; Editing by Stephen Coates Our Standards: The Thomson Reuters Trust Principles.