Police forces say they have received a number of calls reporting what is believed to have been a meteor.
A "huge fireball" was reported travelling from northern Scotland to southern England at about 21:40 GMT, amid fears a plane had crashed.
Police received reports of a "bright light" and "orange glow", but aircraft-related incidents were ruled out.
The Met Office tweeted: "Hi All, for anyone seeing something in the night sky, we believe it was a meteorite."
Meteors are particles from space that burn up as they plummet through Earth's atmosphere, sometimes emitting light, creating a "fireball" effect.
Meteorites are larger, more durable objects that survive heating in the atmosphere and land on Earth. It is not known if that happened on Saturday.
'Is life ending?'
Hundreds of people tweeted about what they had seen and the Kielder Observatory, in Northumberland, described it as a "huge fireball" travelling from north to south over the county.
Gary Fildes, observatory director, who was with a group of about 40 people when they spotted the meteor, said: "We got an incredible view. It was phenomenal.
"They went absolutely mental. I was getting questions about what it is and is it going to end life on Earth? It was massively exciting."
Mr Fildes, who has been an astronomer for 30 years, said he had never seen anything like it and described the experience as "one I'll never forget as long as I live".
Dr David Whitehouse, a science writer, said: "Occasionally you get a very big piece of debris coming into the Earth's atmosphere and this causes a fireball.
"When you see this fireball breaking up, you're seeing the wreckage of a planet that couldn't form properly when the solar system was young and a bit of rock that has been orbiting the Sun for perhaps thousands of millions of years."
'Ball of fire'
Adrian West, of Meteorwatch, said he had seen reports of sightings from Scotland to Devon.
He said he saw the meteor in Berkshire and believed it could have gone down in the English Channel or the Bay of Biscay.
Adam Hepworth, from Helensburgh, in Argyll, told the BBC: "I was leaving work and getting into my car and I noticed a really bright light moving slowly across the sky.
"At first I thought it was a sky lantern but then I realised it couldn't have been due to the speed that it was moving. I then thought perhaps it is a plane that had caught fire.
"I knew it was really odd and sat there for a few minutes just staring at it."
Grampian Police said many people had reported seeing a "flare or a bright object with a tail", while Dumfries and Galloway Constabulary said it had received reports of a "large ball of fire in the sky".
Strathclyde Police said it had been "inundated" with calls, while Lothian and Borders Police also reported taking "quite a lot" of calls.
Durham Police said air traffic control had confirmed there had not been any incidents of aircraft in difficulties.
A force spokeswoman said: "The sightings are believed to be either an asteroid burning out or similar which has been restricted to the upper atmosphere only."
Laura Yusuf, of Mitcham, in Surrey, said she saw the meteor while travelling on the M6.
"It was an amazing sight. Bright orange flames trailing behind it as it slowly burnt itself out," she told the BBC.
Another witness, who called BBC Radio 5 Live's Stephen Nolan programme, said: "I looked up and saw these two huge tails of light coming off it and I thought it was a plane on fire going down into Edinburgh.
"It was massive, there was the red at the back of it, then these two huge white tails and then these blue bits at the very end."