Two people have been taken to hospital after a woman approached them with a bag, leading to them feeling unwell.
Avon and Somerset Police said they were called to reports of a woman approaching people in Stall Street at around 14:30 BST, which led to some people feeling unwell.
Two people were experiencing breathing difficulties and itchy eyes as a result.
A police spokesperson said officers were working with ambulance and fire crews and have “well rehearsed plans for responding to such incidents”.
They said “as a precaution” emergency personnel were wearing protective clothing.
Police were called by the ambulance service just after 15:00 BST.
“Emergency services remain at the scene at the moment, but we hope to be able to reopen the area as soon as possible,” added the spokesperson.