The man they arrested was said to be bearded and in his 30s and was arrested for possession of a knife.
A Met Police spokesman said: "At this time it is too early to understand the motivation so we have not declared this a terrorist incident".
The suspect was detained a short distance from Westminster Bridge near the spot where a terrorist murdered a police officer in the grounds of the Palace of Westminster, which was locked down at about 11.10am.
A man was photographed with his hands cuffed behind his back close to Parliament's Carriage Gates.
Armed police reportedly swooped on the man and discharged a taser before detaining him.
He added: "The officers approached the man in order to speak with him".
A bearded man wearing a grey hooded jumper and black pants was pictured being held against Parliament's railings with his arms behind his back by a police officer.
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Police are "open-minded" over terror being a potential motive after a man was tasered near Parliament after allegedly reaching for a knife. CNN reporters at the scene saw the man being handcuffed and escorted into a police van by armed officers.
"He stared at me and my mate and we walked past him". According to Allen, there were armed police everywhere, calling on the public to move back, which they did, and the whole incident was over within seconds.
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.
"There were about three or four policeman, one of them shouting at the crowd to get back", another witness, who asked not to be identified, told Reuters.
There are no reported injuries and the incident on Friday morning is not thought to be terror related.
No one was injured in the incident.
A review of security at the perimeter of the parliamentary estate was ordered following Masood's auto and knife rampage, which claimed the lives of five victims, including PC Palmer.