A female zookeeper has been killed in a "freak accident" after a tiger entered the enclosure she was in.
Police were called at 11.15am on Monday to reports of a serious incident Hamerton Zoo Park in Huntingdon, Cambridgeshire.
A Magpas air ambulance was on the scene 20 minutes later and visitors were evacuated from the attraction a little before midday on Monday.
The zoo said: "This appears to have been a freak accident.
"A full investigation is currently under way and we hope that more details can be announced as soon as we are able.
"At no point during the incident did any animals escape their enclosures, and at no point was public safety affected in any way.
"All our thoughts and sympathies are with our colleagues, friends and families at this dreadful time."
The zoo confirmed it would remain closed on Tuesday while the investigation continues.
People took to social media to express their condolences following the female keeper's death.
One user posted on Twitter: "Terrible news about the keeper at Hamerton Zoo. Such an awful shock for everyone RIP."
Another wrote: "A zookeeper sadly died earlier today at Hamerton Zoo. So tragic, their life was taken too soon. RIP."
Cambridgeshire Constabulary said the tiger involved was believed to be fine, as Twitter users expressed fears over what would happen to the animal.
The force posted on Facebook: "Officers attended a serious incident at Hamerton Zoo Park, Steeple Girding, at around 11.15am today.
"A tiger had entered an enclosure with a keeper. Sadly the female zookeeper died at the scene.
"At no time did the animal escape from the enclosure.
"The incident is not believed to be suspicious."
The force had earlier denied that any animals had escaped, saying members of the public were safe.
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 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 confirmed that the incident was not terrorism-related.
An EEAST ambulance service paramedic crew and two rapid response vehicles also attended.
Four years ago, Sarah McClay died after being mauled by a Sumatran tiger at South Lakes Safari Zoo in Cumbria.
The zoo was fined £255,000 at Preston Crown Court in June last year following the 24-year-old employee's death in May 2013.
It received an additional £42,500 fine after it also pleaded guilty to other health and safety law breaches when a zookeeper fell from a ladder while preparing to feed big cats in July 2014.
Last October ZSL London Zoo was put on lockdown after Kumbuka the silverback gorilla managed to exit his enclosure through two security doors that had been left unlocked by a keeper.
Armed police were called to the central London attraction and visitors were evacuated when the alarm was raised following the ape's bid for freedom shortly after 5pm on October 13.
It follows the death of endangered gorilla Harambe, who was widely mourned after he was shot when a three-year-old boy fell into his Cincinnati Zoo enclosure in May 2016.