Mayor Rees said: "As you will have heard, a major incident has been declared at Wessex Water’s water recycling centre in Avonmouth.
"Tragically, four people who were working at the site have lost their lives, and the thoughts of our city is with their families at this heart-breaking time.
"This has already been such a challenging year, and this news of further loss of life is another terrible blow. As a city we will mourn for them.
"Local residents may be concerned about safety in the area. Wessex Water, police and fire have assured us the incident is contained, and there is no risk to your safety.
"However we would urge you to keep roads around the site as clear as possible so our emergency teams can do their vital work.
"Thank you to the emergency services for doing their very best in challenging circumstances."