Police said one suspect in his 30s had been arrested on suspicion of possession of a knife “near to the Palace of Westminster”.
“There are no reports of any injuries,” police said.
A commotion was heard outside Parliament shortly after 11.10am, and armed police ran towards the gates to New Palace Yard, the scene of the terrorist attack on 22 March in which PC Keith Palmer was stabbed to death.
Joe Murphy, the political editor of the Evening Standard, said there were shouts of “knife knife knife” and a man was brought down by police with a taser.
The gates to the Palace of Westminster were then shut and journalists inside have been told they cannot leave.
St Stephen’s Gate and Carriage Gate have been locked, with a number of police vehicles and officers gathered in New Palace Yard.
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