Four fire engines and around 25 firefighters were called to reports of smoke issuing the basement of an office block on Great Marlborough Street, in Soho.
Firefighters believe the smoke was due to a small rubbish fire below pavement level caused by the unsafe disposal of a cigarette.
A London Fire Brigade spokesperson said: “If you're a smoker it’s absolutely vital you ensure your cigarette is completely out when you’ve finished smoking it.
“If you don’t, you risk causing a fire which could not only destroy your home, but also cost you your life.”
Around 700 people left the building before the arrival of the Brigade.
The Brigade was called at 1411 and the incident was over for firefighters at 1511. Fire crews from Soho, Euston, and Lambeth fire stations are attending the incident.