Officers with the #London Metropolitan Police were forced to use a Taser on a man close to the Palace of #Westminster as the suspect appeared to reach for a knife.
Police said the man was acting suspiciously at around 11:00 am (1000 GMT) near one of the gates where a militant killed a policeman less than three months ago.
Scotland Yard have not declared a terrorist incident but revealed counter-terror officers will be investigating "given the location, the circumstances and recent tragic events".
He remains in custody at a central London police station, the force added.
Eye-witnesses described seeing a man "tasered" by armed police outside the House of Commons after shouts of "knife, knife, knife".
"You could tell he was suspicious, he was stood there fists clenched. He looked quite an angry geezer", a witness, Bradley Allen, 19, told Reuters. Five, including Masood, were killed in the March attack and dozens were injured.
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'Within seconds there were police all around and they had him pinned down.
"Armed police everywhere, telling everyone to move back, and we all got pushed back".
A Met spokesman confirmed an arrest had been made and there were no reports of any injuries.
London Metropolitan Police said in a statement that they led the suspect away from the scene after questioning him outside the #Houses Of Parliament for approximately 30 minutes.
Ben Glaze, a political reporter from the Daily Mirror says he saw police take some items from the floor around the suspect.
The Carriage Gates entrance to Parliament was closed following the arrest but has since been reopened.
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