Police in Essex are investigating reports that a lion was spotted in a field at St Osyth near Clacton-on-Sea.
The animal was seen near Earls Hall Drive by holidaymaker Bob Martin at about 19:00 BST on Sunday.
Mr Martin said he and his wife Denise saw a large cat and a lion "was the first thing that came to mind".
Colchester Zoo experts have said a definite identification of the animal was not possible based on a photograph taken by a member of the public.
"We believe we saw a large cat looking at a tree... it just sat there looking at us," said Mr Martin.
"Lion was the first thing that came into mind but it was several hundred yards away... it was very impressive."
Mrs Martin added: "We saw this shape in the field, got our binoculars and it was a very, very large cat. At one time it stood up and looked at us, it was a bit spooky as it could obviously hear us, it was just preening itself.
"To start with we was convinced it was a lion but the shape of the ears is slightly wrong, but it was the size of a lion. It had a white chest with like a tan, beige-colour body."
Armed police have been drafted into the area and two police helicopters are searching the area where it was spotted. One of the helicopters is using a thermal imaging camera to try to detect any trace of an animal.
Anthony Tropeano, zoological director of Colchester Zoo who has been at the scene, said: "The photograph shows an animal in a field, it is a sandy brown colour, but the photography is of such poor quality it's not possible for us to say one way or the other what it definitely is.
"There were absolutely no sightings of it during the time I was there. Nothing really to provide any further evidence to suggest it had been there - just the fact of these couple of eye witness account and police had to err on the side of caution."
Residents were advised by police on Sunday night to stay indoors, but police have said: "We want people to go out and enjoy themselves, but if they do wish to go for a country walk and use the fields and footpaths we ask them to be a bit cautious."
Essex Police said about 25 officers were at the scene, including about a dozen specialist firearms officers, and experts from Colchester Zoo were also on hand. They are looking for signs such as pawprints.
Police said all the animals at Colchester Zoo had been accounted for.
The spokesman said a circus had been in the area, at Clacton Airfield, recently but added: "Officers have spoken to the circus and confirmed that they do not have an animal missing, nor do they have any lions."
Local resident Che Kevlin told the BBC he heard a "loud roar" at 22:00 BST. He said that shortly after hearing the roaring sound he got a text from a relative telling him a lion was on the loose in St Osyth.
"It was worrying as we had just been for a walk with the dog. We saw the police helicopter but thought it was just searching for a person," said Mr Kevlin.
"It sounded like the roar of a lion. We have field and wood just behind our fence, so you never know."
There are several holiday caravan parks in and around St Osyth and Sean Williams, who is in a caravan, said he was worried about going out.
Mr Williams, who said he heard about the lion on Twitter, said: "The children are with me and we are quite worried. We were out in fields in St Osyth but we're back in the caravan now.
"We've only seen one police car bombing it down the road. We had a fishing trip planned for tomorrow but we're now debating whether to carry on."
Dave Sparks, who runs the Red Lion pub in St Osyth, said it had been a "surreal" night and told ITV News: "You don't really expect to be told there's a lion wandering around the area.
"No-one really felt there was any immediate danger in the village - a couple of families with young children were a bit worried but after a while they were okay."
Police are asking anyone who sees the animal to call 999.
The news of a potential lion on the loose has captured the public imagination and three Twitter accounts - @EssexLion, @TheEssexLion and @Essex_Lion - were quickly set up.