Smoke billowed into the sky as more than 60 firefighters tackled a blaze at the top of a 33-storey tower block in Canary Wharf.
The top of Barclays building caught fire in the early hours of Saturday morning.
London Fire Brigade said an air conditioning unit was alight.
Sixty firefighters from stations in Millwall and Shadwell were called to the scene at 0.40am.
LFB said it received 20 emergency calls as smoke could be seen billowing across the Canary Wharf skyline.
No injuries were reported and the fire was brought under control shortly before 2am.
