Police searching for a lion in Essex have said they are taking the hunt seriously.

The claims of a potential killer cat on the loose were met with a mixture of shock and disbelief but Essex Police scrambled helicopters with heat-seeking equipment to land around Earls Hall Drive in St Osyth, near Clacton, where the lion was apparently seen.

Initially police warned residents to stay inside their homes as officers began a search for the unusual Most Wanted target. But on Monday police said residents could still enjoy a walk in Clacton's countryside.

By Monday afternoon, police said they had not yet found any proof, such as prints or droppings, to confirm the sighting. A spokesman said between two and six people claimed to have seen the big cat but there had been no further sightings since Sunday. The police hunt is likely to be scaled down by nightfall if no evidence emerges, the spokesman added.

The police enlisted the help of experts from Colchester Zoo who believe the sighting to be genuine after viewing a photograph of the creature taken by a local.

As well as a heightened police presence and helicopter search, zoo workers said to be armed with tranquilliser guns also joined armed officers. Officers were returning to areas where the animal was apparently spotted to search for clues such as pawprints.

Mystery surrounds where the lion might have come from. Police ruled out Colchester Zoo as all its animals were accounted for. Rumours spread that the lion might have fled a circus performing in the Essex area, but they are yet to be substantiated.

Denise Martin, 52, a warehouse operative, was the first to spot the suspected lion on Sunday evening from the windows of her caravan on the site at Earls Hall Farm. Mrs Martin, from Canvey Island, was spending the Bank Holiday weekend at the site with her husband, Bob, 51, a professional driver, her brother, David Wright, 57, a lorry driver, and his wife, Sue, 58.

"I was looking out of the window and we saw smoke. When the smoke cleared I could see this shape in the field, so I got the binoculars out. We had a look and it looked like a lion. I said to my husband 'What do you make of that?' He said 'That's a lion.' Five or 10 minutes later we got my brother and my sister-in-law to have a look. They came and looked at it and said 'It looks like a lion.' My husband phoned the police."

She said the lion was tan coloured with a white chest, adding: "We weren't scared at all - it was excitement. You don't often see something like that in the wild. One time it sat up and looked at us and we could see its ears twitching. It knew we were there and it sat down and started cleaning itself."