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.