Visitors have been evacuated from Hamerton Zoo as police and air ambulance were spotted outside the attraction.
People took to social media to tweet about the closure in Huntingdon, Cambridgeshire.
Chris Hubbard posted: "Police and air ambulance called to @HamertonZoo and guests are being cleared out. Hope everyone's okay."
He also tweeted a picture of the scene which appeared to show an air ambulance and emergency services on the scene.
Jessica Russell said on the site: "I shouldn't leave the house... Try to go to @HamertonZoo and all hell breaks loose. Evacuated and air ambulance/police/ambulances."
Cambridgeshire Constabulary said no animals had escaped and members of the public were safe.
In a statement on Facebook the force said: "We were called at around 11.15am to reports of a serious incident at Hamerton Zoo Park, Steeple Gidding.
"Officers attended the scene along with ambulance crews and Magpas (air ambulance).
"We are unable (to give) further information available at this time, however, we can confirm that no animals have escaped and members of the public are safe.
"We will provide a further update when we are able to."
Eyewitness Jeff Knott, from Cambridgeshire, said staff had been "a real credit" to the zoo during the evacuation.
The 32-year-old told the Press Association: "We had been in the zoo since about 10.30am and heard/seen nothing until asked to leave about 11.45am.
"Staff were very calm and professional. All visitors around us were leaving in a very calm manner - no running, shouting or anything similar."
Antonia Brickell from Magpas air ambulance said they arrived at the zoo at 11.34am after being called to a "very serious incident".
She denied reports that visitors had been "running in terror", adding: "The visitors were safely evicted from the zoo. No animals escaped the enclosure, no children were injured."
She also said reports that a male member of the public had been injured were not accurate, and confirmed that the incident was not terrorism-related.
An EEAST ambulance service paramedic crew and two rapid response vehicles also attended.