Firefighters have been investigating at incident at Canary Wharf's One Canada Square.
Those hoping to enter the tower were prevented from doing so as the London Fire Brigade responded to a call made at 9.50am.
A spokesperson told The Wharf they were contacted after the fire alarm system was set off.
Investigations have now finished.
Office workers facing a late start to the day crowded outside bars including Smollensky's, in Reuters Plaza while the lobby lay empty.
The Wharf's deputy editor Jon Massey, one of the group gathered outside, said: "It's very tense."
Meanwhile, some of their colleagues - including more of The Wharf team - were advised to remain in E14's tallest building and await further news.