The US embassy was placed on lockdown during a security incident after a false alarm, following reports of a suspicious package.
People inside were told to stay away from the windows and sniffer dogs have been seen outside the building.
The embassy, based in the Nine Elms area of London, has since said it is back to operating normally and thanked police for their response.
They said on Twitter: "The US Embassy is back to normal business operations. Local authorities investigated and cleared a suspicious package outside the Embassy.
"Thanks to @metpoliceuk for your swift action, and thanks to all visitors for your cooperation and patience at this time."
Armed police were seen patrolling inside the building as they instructed those inside. At least two fire engines were seen in the area along with police vehicles.
The incident was stood down at 11.07am as it was declared a false alarm.
Eyewitness Aro Korol was inside the building at the time and told the Mirror that security was "assessing the situation" and that those inside had been escorted to another part of the building.
Another said: "We don’t know much other than there’s a situation ongoing and stay away from the windows."
One Twitter user said at the time: "Something is happening at the US Embassy London.
"I am inside, They told us to move away from the windows. Ongoing situation outside."
The Met Police has said it was aware of the incident and have been approached for comment.
