A man was Tasered inside the grounds of the Houses of Parliament where PC Keith Palmer was knifed by the Westminster Bridge attacker.
The young man wearing a green bomber jacket over a bright orange African and baseball cap was brought to the floor and arrested in New Place Yard.
The latest security scare happened just after midday at the entrance into the Palace of Westminster near Big Ben.
Pc Palmer was killed as he challenged Islamist terrorist Khalid Masood in New Palace Yard in March last year.
An eyewitness said: “I saw a guy fall to the floor and on on the floor surrounded by armed officers. He was shouting something, it sound Arabic.
“He had been Tasered, you could see the wires. I did not hear the police shouting anything, but that could be because of all the commotion.
“They handcuffed him and got him to his feet, then unzipped his jacket to see if he had anything hidden on him
“Everybody was just watching, surprised that something like this was happening.
“Two armed police cars, BMW X5s turned up and armed police came out of the grounds and told us to get back.
“They then set up cordons around the entrance.”
By Tony Whitfield