Police have evacuated people from shops in Uxbridge High Street - including intu Uxbridge - while they deal with a security alert.
Officers were called at 12.14pm to High Street, Uxbridge. They are still at the scene and have blocked off a section of Vine Street.
Shoppers and staff at shops along the High Street have been told to leave the area by police and many people are congregating outside the Civic Centre.
Yvonne Janis, 79, from Hayes, was among those shopping when the alarm sounded.
She said she saw police sniffer dogs. "People were very calm," she said. "I was just thinking whether it was worth coming out today, but [the police] are only doing their job."
Sandra Jones, 62, of Windmill Road, Ruislip Manor, was among the crowd waiting outside.
She said: "All the police were here. I was waiting to get to my car on my way home when I saw all the police here.
"I wouldn't have thought there would be such a police presence."
The incident came just over an hour after the annual Remembrance Day ceremony outside Hillingdon Civic Centre in the High Street at which members of the armed forces and the public mingle with Hillingdon Council staff and guests to mark the anniversary of the end of the First World War.
The traditional two-minute silence had been heralded by the boom of a maroon rocket.
Shoppers would have heard it loud and clear from Rockingham Recreation ground, the closest of 11 sites where the rockets are launched.
A number of bus routes are diverted while the incident is in place.
People commenting on the incident on Twitter are calling it a bomb scare but police have not confirmed this.
Vine Street was closed off by police in October after a suspicious item was discovered.
In 2011, police closed off the town centre and bomb experts detonated a hatchback after reports it contained a suspicious package.
Meanwhile, the Evening Standard has reported that Met Police officers blew up a handbag outside the Imperial War Museum in Lambeth this morning (Tuesday) shortly before 11am.
UPDATE: A 40-year-old man was arrested by police on suspicion of making a threat to cause an explosion.
He was taken to a West London police station where he is being detained.
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