The Shard in London has been evacuated after emergency services were called to reports of smoke coming from the basement.
London Fire Brigade said that seven fire engines with 50 firefighters were at the scene after it was called to investigate at 10.30am this morning.
Around 900 people were evacuated from the building, fire service officials said, adding that a full evacuation of the Shard's 72 floors had been called as a precaution, and that there was no suggestion at this stage of there being a fire.
The fire service later said that smoke "appeared to be dissipating" and that crews were investigating what caused the alarm.
The official Twitter account for the tower, @TheShardLondon, said emergency services were responding and that "evacuation procedures have been followed and we are ready to assist as needed".
Witnesses praised the fire services' rapid response time on Twitter, with engines coming from stations including Soho, Lambeth and Whitechapel.
A spokesperson said that the large number of firefighters was normal for an incident at a building of the Shard's size.
It is believed to be the first evacuation of the tower, western Europe's tallest standing at more than 1,000 feet.
Police and paramedics were also called to the building, though a statement from the Metropolitan Police said there were no reported injuries or arrests at this time.
Roads have been closed around the area, including a police cordon reportedly set up at St Thomas Street. London Fire Brigade said public transport services at London Bridge station were unaffected.