British police said Monday they were investigating reports of a lion being spotted in a field near a seaside town in the eastern county of Essex.
Armed officers spent the night scouring the area, while a police helicopter with heat-seeking equipment was deployed to help find the animal, alongside zoo workers armed with tranquilliser guns.
Officers were contacted shortly before 7pm (1800 GMT) on Sunday with a sighting by holidaymakers of the animal sitting in grassland in the village of St Osyth near Clacton-on-Sea.
Essex Police said their officers had been working with experts from nearby Colchester Zoo who believe the reports to be genuine after being shown a photograph from a member of the public.
A police spokesman said: "Public safety is our priority which is why we are taking the sighting and all associated evidence seriously.
"We have an increased police presence in the area which will offer reassurance to local residents, motorists and holidaymakers.
"For those who wish to take a walk in the countryside surrounding Clacton, then we would just ask them to be extra vigilant and cautious."
The spokesman said that a member of the public had reported finding a large footprint or pugmark.
He said around a dozen officers were still involved in the search on Monday.
Local resident Che Kevlin said he heard what he believed to be a roar whilst out walking his dog on Sunday night.
"I heard a loud roar at 10pm. It sounded like a lion," he told BBC television.
"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."
Police are trying to establish where the lion may have come from and have urged anyone who sees it to call the emergency number 999.
A police spokeswoman confirmed the animal had not escaped from Colchester Zoo, as all its animals had been accounted for.
A circus based near Clacton-on-Sea early in August said it did not keep any lions.