A major hospital in London was evacuated and one staff member was injured following a chemical explosion.
Around 150 people at Guy's Hospital in central London were told to leave the site immediately.
It is understood that an engineer "accidentally mixed up chlorine and sulphuric acid" in the basement of the hospital.
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Images show fire engines at the scene and crowds of people gathering outside Guy's Hospital
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One member of staff was injured following the chemical reaction and a number of people who helped the injured person have been treated for the inhalation of chlorine gas.
Patients, visitors and staff were evacuated from the basement and ground floor levels.
According to a police officer at the scene: “There was a chemical leak in the basement of Guy’s hospital. It’s just reopened now.
"One of the engineers accidentally mixed chlorine with sulphuric acid, and it totally exploded," they told The Sun.
Another officer added: "There’s been a chemical spill in the basement that was potentially quite dangerous.
"But it’s all been cleared and it’s absolutely no risk to anybody."
London Fire Brigade has confirmed they are responding to reports of a chemical incident
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Images showed fire engines at the scene and crowds of people gathering following reports of the possible explosion.
Crews were carrying out operations to ventilate the building.
London Fire Brigade (LFB) received the first report of the incident at 8.49am, dispatching crews from Whitechapel, Dowgate, Euston, and nearby stations to the scene.
A spokesman for the LFB said: "Firefighters responded to reports of a chemical incident at Guys Hospital, SE1, today.
"The incident is believed to have been caused by the mixing of chemicals inside a plant room which produced a chlorine gas.
"One person was injured as a result of a chemical reaction. Several people, who came to the aid of the person, have been treated for the inhalation of chlorine gas.
"Firefighters have carried out a sweep of the area to check for no elevated readings of chlorine gas. The building has also been ventilated.
"Crews also supported with the precautionary evacuation of the basement and ground floors of the building."
Around 150 people at Guy's Hospital in central London have been told to leave the site immediately
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Two fire engines, two Fire Rescue Units, a Command Unit, and specialist hazardous materials officers were in attendance.
In a statement, London's fire service said: "Firefighters responded to reports of chemical incident at Guys Hospital.
"Two fire engines, two Fire Rescue Units, a Command Unit and specialist hazardous materials officers attended the scene.
"Crews carried out operations to ventilate the building and evacuated staff, patients and visitors from the basement and ground floor levels of the hospital.
"The Brigade was first called about the incident at 0849, with crews from Whitechapel, Dowgate, Euston and surrounding fire stations have been sent to the scene. The incident was over for firefighters by 1109."
The incident is now understood to have been resolved.
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