A police cordon is in place in Salisbury close to the restaurant where Sergei and Yulia Skripal had lunch before they fell ill.
Wiltshire Police said they are “taking highly precautionary measures” in dealing with an incident involving a man in his 30s because of the ongoing situation in the city and nearby Amesbury, in Wiltshire.
But the force said residents should not be alarmed and there is nothing to suggest there is any risk to the wider public.
An eyewitness, who asked not to be named, told the Press Association the man involved was a “local homeless guy” who was “sat on the floor, completely conscious, talking”.
He said he saw a paramedic at the scene climbing into a chemical suit while another “was shouting at the man to stay put and stay on the floor”.
“All the police officers had rubber gloves on,” he added.
Publisher George Walkley, 41, arrived on the scene at around 8pm.
He said: “I was walking through the city centre and saw emergency vehicles and they had sealed off a stretch of Castle Street in Salisbury.
“There was a very heavy police presence there along with the fire service and ambulances.
“Just inside that cordon between Zizzi and Tesco they had a green tent thing set up.”
The Skripals ate at the restaurant shortly before they were found unconscious on a nearby bench on March 4 after being poisoned with Novichok.
Charlie Rowley, 45, remains seriously ill in hospital, and his partner Dawn Sturgess, 44, died after the pair collapsed at his home in Amesbury, on June 30, having been exposed to the nerve agent.
Wiltshire Police said in a statement: “We are currently dealing with an incident in Castle Street, Salisbury, involving a man in his 30s.
“Given the ongoing incident in Salisbury and Amesbury, we are taking highly precautionary measures to ensure public safety.
“While we do not want people to be alarmed hope you understand the reasons behind this and we will update you when we have any further information.
“At the moment there is nothing to suggest any wider risk to the public and we would ask people to avoid the immediate area and be respectful of the cordon which is in place.”