Police said none of the zoo's animals had escaped and the public were not in danger, but refused to give any more details.
Pete Davis, who had been visiting the 10ha zoo with his family, said he believed a young keeper may have been injured after running into the tiger enclosure to help a colleague.
He said he had seen the young woman run into an enclosure after hearing shouts from a colleague. Then he had heard screams.
Davis, 55, said: "There's no doubting it was a girl's scream and something terrible had happened. It sounds like a tiger turned on her."
Another keeper then ran out and ordered visitors to run.
"A group of us ran into the keepers' rooms for about 10 minutes until a keeper said we could leave.
"At this point we understood by the reactions of the staff that something was going on inside the enclosure. We could see staff members on the other side of the fence with pieces of meat trying to get whatever's attention. It was heartbreaking seeing them trying to help."
The zoo near Sawtry in Cambridgeshire, has about 500 animals, including tigers, cheetahs and a number of other wild cats, including lynxes, servals and oncillas. The zoo also has wolves, monkeys, raccoons and sloths.
A spokesman for Cambridgeshire police said they were called to the zoo about 11.15am.
"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."
He added: "There is nothing ongoing at the location now that would require anyone to be concerned."
