Around 900 people were evacuated from Western Europe’s tallest building as firefighters investigated reports of "slight'' smoke.
London Fire Brigade said there was no evidence of a fire in the 1,004ft Shard, near London Bridge, adding that the smoke was dissipating.
A spokesman said the whole building was evacuated as a precaution and no injuries had been reported.
He said the presence of seven fire engines and around 50 firefighters was usual for that type of building.
Scotland Yard said no arrests had been made and inquiries were going on into the source of the smoke.
London Ambulance Service was also called out but said “there are no patients”.
London Fire Brigade station manager Keeley Foster said crews had been called to "slight'' smoke in the Shard's basement.
Asked about the evacuation, she said: “Once the alarm sounded in the building, the management put procedures into place for the evacuation process.
“As we arrived, the commander at the time decided that as a precaution the entire building was to be evacuated and crews quickly worked alongside building management to make sure that occurred.
“Evacuating such large numbers of people can impede crews if there is a fire apparent but because it has been practised a number of times and we have worked alongside building management on a number of exercises, it was quite a quick and easy process.”
Ms Foster said an investigation was under way to establish the cause of the scare.
She said several potential factors could have caused smoke without fire, such as an electrical fault.