Shoppers posted pictures of the ordeal on social media, but the reason for the evacuation is unknown.
One shopper said the “smoke curtains” were down.
One shopper said 'smoke curtains' had come down
Thousands were evacuated from the centre in east London after reports of a fire breaking out.
Concerned shoppers caught up in events posted pictures of the operation as alarms blared and stores were shut.
At least two reports on social media claimed a fire had broken out.
Westfields Shopping Centre in London
One picture on Twitter showed Nando’s customers still clutching their plates as they left the building.
One Twitter user wrote: "There’s a fire in Westfield and we’re being evacuated.”
Another wrote: "Just evacuated from Westfield. Alarms going off, police and lots of people wonder what happened???."
A third witness added: "On our way to get our train, walking through Westfield and we all get evacuated because there's a fire. Crazy."
Westfield Shopping Centre in east London (archive image)
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A fourth wrote: “On our way to get our train, walking through Westfield and we all get evacuated because there’s a fire. Crazy.”
Stand-up comedian Ariane Sherine was also caught up in the ordeal.
She tweeted: "In Westfield Stratford with Lily. Fire alarm in whole centre.
"Shops evacuated, shutting up; cordoned off area, can't get to Tube. Bit scary!"
Stand-up comedian Ariane Sherine was caught up in events at Westfield
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A tweet was later posted on the official Twitter account for Westfield shopping centre, which confirmed the evacuation was sparked by a leak and not by fire.
It read: "There was a partial evacuation this evening due to a leak. The issue has now been resolved and we are open for business as usual."
Others expressed concern that the evacuation was poorly managed.
One shopper said: "That evacuation process was a shambles. No info, no staff, no clue.God help customers if something serious actually happened."
Westfield Stratford's Twitter post
A security officer denied to Express.co.uk that the centre had been evacuated.
An investigation is underway into what caused the alarm to sound but a cordon around an area in the middle of the shopping centre has since been lifted.