Whitehall and surrounding roads were closed by police as officers investigated a “suspicious item”.
The Metropolitan Police said its officers were called at 12.01pm.
Roads around the thoroughfare were blocked before investigators gave the all-clear at about 1pm.
In a tweet, the Met's Westminster division said: “Temporary road closures in place around Whitehall whist suspect item assessed. Police attended at 1201hrs today. Specialist officers are now in attendance. Thank you for your patience”.
Later, it added: "Happy to update the item concerned has now been fully assessed and deemed non-suspicious. Roads will be reopened shortly. Thanks again for your patience and cooperation".
It came as voters went to the polls in European parliament elections.
Reports from the area suggested people were not being allowed out of 10 Downing Street or the Cabinet Office during the investigation.
Video appeared to show bomb disposal experts accompanied by sniffer dogs and a robot near the Cenotaph.
