Parliament was evacuated on Monday morning following a suspected security alert.
According to witnesses at the scene, an alarm sounded and the evacuation began at around 10.45am.
The London Fire Brigade denied reports the alert was a fire alarm and said they have not been called to the scene.
A photo from Westminster showed crowds of people outside the Houses of Parliament as the evacuation began.
Several people on social media reported the alert was sparked by reports of an unattended bag.
Matthew van Rooyen, who was at the scene, said passholders were allowed back into Parliament around 15 minutes after the alarm sounded.
Anne Alexander, who is the political producer for ITV programme Good Morning Britain, said she reported an unattended bag outside Parliament at around 10.35am.
A short while later, she tweeted: “It’s all over! We’re being allowed back to the old bit.”