Police were made aware of a planned protest outside a Covid-19 vaccination centre in Churchill Square in Brighton on Monday (August 30).
Demonstrators started to gather from around 9.35am and by midday around 80 people were gathered on private land outside the shop unit formerly occupied by Topshop.
Police officers and PCSOs attended to engage with protesters and facilitate a peaceful demonstration while minimising disruption for members of the public.
A device believed to be a smoke bomb was set off at around 1.35pm and the NHS took the decision to temporarily close the vaccination centre shortly afterwards.
The protest moved to public land in Russell Place, opposite Churchill Square, and continued until 5.50pm.
Chief Superintendent Justin Burtenshaw said: "We recognise the public's right to protest and our officers will work with organisers to ensure the safety of demonstrators and the general public, as well as to minimise disruption.
"Enquiries are currently ongoing into Monday's demonstration, particularly around the circumstances that led to what is believed to be a smoke bomb being set off. No arrests were made.
"We would encourage anybody who has any information which could help with the investigation to contact police online or by calling 101, quoting serial 048 of 30/08."