An offence of violent disorder contrary to s 2(1) of the Public Order Act 1986 (UK) — similar to unlawful assembly, contrary to common law — requires only that three or more people are in the same place at the same time. It’s not necessary to prove they deliberately acted in combination with each other.
In R v NW  EWCA Crim 404 the UK Court of Appeal rejected a 15-year-old schoolgirl’s appeal against conviction for violent disorder after she and a friend fought police officers while an unruly mob surrounded and stopped the police from arresting the girls.
When the cavalry arrived, the crowd lost interest and disappeared.
The trial judge contrasted the offence with riot, which requires a common purpose. The Court of Appeal approved his interpretation.
 That being so, we think that the expression “present together” means no more than being in the same place at the same time. Three or more people using or threatening violence in the same place at the same time, whether for the same purpose or different purposes, are capable of creating a daunting prospect for those who may encounter them simply by reason of the fact that they represent a breakdown of law and order which has unpredictable consequences. We are unable to accept that the phrase requires any degree of co-operation between those who are using or threatening violence; all that is required is that they be present in the same place at the same time. The section is concerned with public disorder and is deliberately worded in a way that is apt to apply to anyone who uses or threatens violence of the requisite nature in a particular context, namely, in a public place where others are engaged in the same activity. We think that the requirement that the conduct of the participants taken together should be such as to cause members of the public to fear for their safety was included in order to direct attention to the overall effect of what may otherwise be unrelated acts or threats of violence.
Here, unlawful assembly remains a common law offence and requires the prosecution prove an element of intention:
- Three or more people assembled together;
- Intending to:
- Commit, or encourage the commission of, a crime by open force; or
- Carry out any common purpose, whether lawful or unlawful, in such a manner to give firm and courageous people in the neighbourhood of the assembly reasonable ground to apprehend a breach of the peace resulting from that common purpose.
That differs from the UK’s statutory provision.
Probably the closest statutory offence here is riotous behaviour in a public place, contrary to s 17(1)(d) of the Summary Offences Act 1966.
Unlawful assembly remains triable summarily because s 28(1) of the Criminal Procedure Act 2009 provides summary jurisdiction for common law offences punishable as level 5 or 6 offences, and s 320 of the Crimes Act 1958 lists unlawful assembly as a level 6 offence.