Police confirmed that they arrested a man in his late 20s.
The driver was arrested by armed officers and taken to a south London police station, where he remains in custody.
Sky News UK and other media outlets report there is a heavy armed police presence outside the building after the auto crash.
Masood was then killed himself after armed police arrived at the scene.
As CBS News correspondent Debora Patta reports, emergency services treated three people at the scene of Tuesday's apparent attack, but there were no life threatening injuries; probably thanks to special anti-terrorist barriers erected at strategic locations after a similar incident a year ago in which a terrorist rammed his vehicle through pedestrians on Westminster Bridge before stabbing people, leaving a total of five dead.
"Officers remain at the scene and cordons are in place to assist the investigation".
"I was told to move and started to run", he said. ISIS claimed the 22-year-old British perpetrator of Libyan descent as one of its own.
Witnesses said the vehicle picked up speed after hitting some cyclists and crashed into a safety barrier.
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He said that based on what investigators know so far, the suspect does not appear to be someone who was previously known to British counterterror or intelligence agencies.
Two people were taken to the hospital and one person was treated at the scene of the crash.
Police also said on Tuesday they were carrying out searches at two addresses in the city, as well as an address in nearby Nottingham.
She said: "I was walking on the other side [of the road]".
A silver Ford Fiesta sped down the street, striking a group of cyclists before smashing into a concrete barrier created to thwart vehicle bombs. I heard some noise and someone screamed. It was driven around central London streets before the attack was launched at 7.30am.
There will be an increased police presence in Scotland, England and Wales following the incident at Westminster.
At the time of publishing, no terrorist organization has claimed responsibility for the attack.