The horrific incident took place at around 11.15 this morning, as panicked zookeepers evacuated the park, near Huntingdon in Cambridgeshire.
Initially zoo visitors believed a tiger had escaped its enclosure, although officials insisted this was not the case.
A zoo spokesman said: "A full investigation is currently underway 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 familes at this dreadful time.
"The park will be closed from tomorrow 30th May, and we will give more information as soon as we can."
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Emergency services at the scene in Hamerton Zoo Park
Stunned eyewitness Pete Davies, 55, said the zoo keepers were "throwing meat over the enclosure to entice the tiger away".
The 55-year-old added: "They shouted stuff. It went on for too long. Then we were ushered out and left.
"We didn't see anything because it happened behind a mound of grass. It seems like a tiger has turned on a zoo keeper.”
He told the Cambridge News how he heard a "blood-curdling scream" from another zookeeper who ran into the enclosure to help her colleague.
Hamerton Zoo Park's website features several prominent images of tigers
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Another visitor recalled how he ran from the tiger area of the zoo, fearing one had escaped.
Kevin Fernandez wrote on Facebook: “Never run so fast in my life. They closed the zoo ambulance turned up, I think someone got bitten.
“We just saw everyone running out of the tiger bit and all the staff was shouting: "Run, get out". We had to leave the zoo, loads of ambulances turned up."
A serious incident has occurred at Hamerton Park Zoo
A spokesman for Cambridgeshire Constabulary said: "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 zoo keeper died at the scene.
"At no time did the animal escape from the enclosure. The incident is not believed to be suspicious."
Hamerton Zoo Park confirmed the tragic news about the zookeeper's death
The zoo houses some 500 animals including a collection of white tigers and Malaysian tigers.
The East of England Ambulance Service said: "The East of England Ambulance Service Trust was called to a serious incident near Sawtry, Cambridgeshire at 11.21 this morning.
"An ambulance, two rapid response vehicles and Magpas Air Ambulance attended."
A spokeswoman for Magpas Air Ambulance said: "At 11:34am today, Magpas Air Ambulance landed at Hamerton Zoo in Cambridgeshire, after the Magpas enhanced doctor and paramedic team was called to a very serious incident.
"An EEAST Ambulance Service paramedic crew and two rapid response vehicles were also in attendance, alongside the Police."