In the UK, though the page I linked is a bit misleading:
Section 14 of the Public Order Act: - Public Assemblies
"As with Section 12, the senior officer may impose conditions on public assemblies, which he considers are reasonably necessary to prevent serious public disorder etc. Unlike Section 12, the conditions he may reasonably impose are in this case limited to specifying:
a) the numbers of people who may take part,
b) the location of the assembly, and
c) its maximum duration.
An assembly is defined by Section 16 of the Act as consisting of two people or more"
In effect, the police can limit the size, location and duration of an assembly at their discretion. That is, they can end it, make it so small it doesn't matter or move it to somewhere it doesn't matter.
Section 14 of the Public Order Act: - Public Assemblies "As with Section 12, the senior officer may impose conditions on public assemblies, which he considers are reasonably necessary to prevent serious public disorder etc. Unlike Section 12, the conditions he may reasonably impose are in this case limited to specifying: a) the numbers of people who may take part, b) the location of the assembly, and c) its maximum duration.
An assembly is defined by Section 16 of the Act as consisting of two people or more"
In effect, the police can limit the size, location and duration of an assembly at their discretion. That is, they can end it, make it so small it doesn't matter or move it to somewhere it doesn't matter.
Effective assembly control.