If an authorised public assembly is held substantially in accordance with
the particulars furnished with respect to it under section 23 (1) (c) or, if
those particulars are amended by agreement between the Commissioner
and the organiser, in accordance with those particulars as amended and
in accordance with any prescribed requirements, a person is not, by
reason of any thing done or omitted to be done by the person for the
purpose only of participating in that public assembly, guilty of any
offence relating to participating in an unlawful assembly or the
obstruction of any person, vehicle or vessel in a public place.