Visitors have been evacuated from Hamerton Zoo in Cambridgeshire after an air ambulance was called to a "very serious incident".
Police quickly denied reports on social media that a tiger had escaped from its enclosure but have not released further details about what caused the site's closure.
Cambridgeshire Police said on Twitter they attended an incident at the 25-acre Hamerton Zoo Park near Huntingdon on Monday morning. A spokesperson confirmed no animals had escaped.
Unconfirmed reports said the incident may have been centred on the tiger enclosure and involving a member of the park's staff.
A statement released by the force said officers were called at around 11.15am to reports of a "serious incident".
"We can confirm that no animals have escaped and members of the public are safe," a statement said.
A spokesman for Cambridgeshire Police said they had suspended contact with the media while they investigated and would release more details later.
Hamerton Zoo Park houses a variety of exotic animals, including Malaysian tigers, cheetahs and lemurs.
The zoo issued a warning to visitors in December last year after a visitor to the park scaled a metal fence barrier.
A spokesperson for the Mid Anglia General Practitioner Accident Service (Magpas) air ambulance said: "At 11:34am today, the air ambulance landed in a public place near Sawtry in Cambridgeshire, after the Magpas enhanced medical team were called to a very serious incident.
"An East of England Ambulance Service paramedic crew and two rapid response vehicles were also in attendance, alongside the police."
The zoo will remain closed for the rest of the bank holiday.
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."
On Saturday, the attraction announced it had finished work on two new enclosures. “Our latest development here at Hamerton Zoo Park…[was] finished yesterday allowing viewing of our Greater Flamingo flock, and one Cassowary enclosure,” the park’s website said.
