The public were warned to secure their homes and avoid what has been dubbed "the beast of Barnet" after two sightings in the South Mimms area on Friday evening.
Despite using a Metropolitan Police helicopter equipped with thermal imaging cameras and increased patrols on the ground, the authorities have so far been unable to corner the animal.
An animal, said to be "a puma-style" big cat larger than a Labrador, with a sandy coat and a long tail ringed with black at its tip, was first seen in the area in woodland eight years ago but rarely has a sighting caused as much disruption.
There were road diversions and officers patrolling streets in South Mimms and Potters Bar with loud-hailers warning the public to lock doors and windows. Police also called in the Royal Society for the Prevention of Cruelty to Animals, the Royal Veterinary College and London Zoo. A police spokeswoman said the sightings were taken seriously because this time the animal was spotted close to a residential area.
However, one big cat expert last night had reservations about the police theory that a "puma-style animal' was on the loose. Nick Lindsay, curator of Whipsnade Wild Animal Park, said there was no big cat that fitted the description given by police.
He said: "A puma is twice as big. What is interesting though is that these sightings tend to coincide with media reports or television programmes dealing with the subject."
Last week saw two such programmes on television. On Wednesday, The X- Creatures on BBC1 dealt with mysterious wild cats. And on Friday - the first time the "beast of Barnet" was spotted - the vet's drama series Noah's Ark dealt with the escape of a leopard from a private zoo.