Visitors were asked to leave the park, located near Huntingdon in Cambridgeshire, at around 11.15 this morning.
Authorities have not disclosed the specific reason, stating only a "serious incident" had occurred.
One witness described running from the tiger area of the zoo, fearing one had escaped.
The zoo denied an animal had escaped, but Kevin Fernandez described the scene on Facebook: “Never run so fast in my life .. they closed the zoo ambulance turned up I think someone got bitten.
“We just see everyone runnin out of the tiger bit and all the staff was shouting run get out .. we had to leave the zoo .. a boy was missing loads of ambulances turned up .. not quite sure what happened one of the tigers got out tho (sic).”
Other social media users also said they feared a tiger had escpaed from the zoo, which houses some 500 animals including a collection of white tigers and Malaysian tigers.
TWITTER / HEIDI MAURY
Emergency services at the scene in Hamerton Zoo Park
Police urged calm and said the incident was not ongoing and did not "require anyone to be concerned".
In a tweeted statement, they said: “We attended an incident at Hamerton Zoo Park today. We are unable [to provide] further info at this time but we can confirm that no animals have escaped.”
A spokesperson for the zoo refused to go into further detail into what the incident was, but denied a tiger had escaped.
When asked by Express.co.uk whether the incident involved an animal, or whether a visitor had entered an enclosure, the representative said he was not at liberty to say.
He said a statement will be issued by the park sometime this afternoon or early evening.
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A serious incident has occurred at Hamerton Park Zoo
A spokesperson for Cambridgeshire Constabulary said: “We were called around 11.15 to reports of a serious incident at Hamerton Park.
“Officers attended the scene along with ambulance crews and MAGPAS [Air Ambulance].
“There is no further information at this time but we will provide an update when we are able to do so.
“There is nothing ongoing at the location now that would require anyone to be concerned.”
HAMERTON ZOO PARK
Hamerton Zoo Park's website features several prominent images of tigers
Hamerton Zoo Park are yet to explain the specifics of the incident today
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 (May 29).
"An ambulance, two rapid response vehicles and Magpas Air Ambulance attended."
MAGPAS Air Ambulance tweeted this afternoon: "To confirm, everyone was safely evacuated from Hamerton Zoo, we can also confirm that no animals escaped and no children were injured."
A spokeswoman for MAGPAS said earlier today: "At 11:34am today (29th), 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."