3. Unlawful assembly (S.18 Public Order Ordinance)
An unlawful assembly is when 3 or more people assemble together and behave in a disorderly, intimidating, insulting or provocative manner that causes a reasonable fear that a breach of the peace may result of that behaviour. The fear of a breach of the peace may be the intention of the people behaving in that manner, or it may be a likely reaction of other people in the nearby the people behaving in that manner. A person convicted with taking part in an unlawful assembly is liable to imprisonment of 5 years on indictment, or a fine of $5,000 and imprisonment of 3 years on summary conviction.
The Court of Appeal has laid down the sentencing principles of the offence of unlawful assembly: If the case is of a relatively minor nature, such as when the unlawful assembly was unpremeditated, small in scale, involving very little violence, and not causing any bodily harm or damage to property, the court may give proportionally more weight to such factors as the personal circumstances of the offender, his motives or reasons for committing the offence and the sentencing factor of rehabilitation while proportionally less weight to the sentencing factor of deterrence; if the case is a serious one, such as when the unlawful assembly involving violence is large-scale or it involves serious violence, the court would give the two sentencing factors, namely punishment and deterrence, great weight and give very little weight or, in an extreme case, no weight to factors such as the personal circumstances of the offender, his motives or reasons for committing the offence and the sentencing factor of rehabilitation. (Secretary for Justice v Wong Chi Fung  HKCFA 4, )
It does not matter whether the assembly started out as a lawful assembly. If 3 or more people in that assembly acted in a disorderly, intimidating, insulting, or provocative manner together and cause a reasonable fear that a breach of the peace may result, it will become an unlawful assembly.
“Three or more persons, assembled together”
This element reflects the ‘corporate nature’ of this offence. The unlawful assembly is made up of those who behave in the prescribed manner. Therefore, if there was only one person out of those who assembled together conducted himself in the prescribed manner he could not be guilty of this offence.
Where a group of people assembled at a location and only three or more amongst them behave in the prescribed manner, it is they, not other members of their group who do not conduct themselves in such manner, who become an unlawful assembly.
There must be sufficient connection between the conducts of those who behave in the prescribed manner to justify having them considered together. Therefore, if three persons in a lawful assembly committed acts of the prescribed nature at different parts of the place of assembly for different purposes, sparking off different incidents, involving and affecting entirely different mix of persons, there would not sufficient nexus to turn these independent acts into an unlawful assembly. But “sufficient nexus” is only an “important factor” in considering whether those persons were acting together, and evidence of a common purpose is only “the best or one of the most important pieces of evidence” to prove that they were so acting together. A common “purpose” does not require a “common motive”, for example the common purpose to attack police officers qualifies as a “common purpose”.
“Disorderly, intimidating, insulting or provocative manner”
What amounts to “disorderly, intimidating, insulting or provocative manner” is a matter of common sense and ordinary meaning of these words. It depends on the time, place and circumstances of the conduct in question.
“Disorderly conduct” has been interpreted as “unruly or offensive behaviour”, “rough or aggressive behaviour”, or “acting in a way which disrupts public order or is against morality”. It does not need to involve any violence or cause serious disruption of public order.
The fear is not about fear as to anyone’s own safety or security. Instead, it is a reasonable fear that a breach of the peace will result. (HKSAR v Leung Kwok Wah  5 HKLRD 556)