A man was reportedly tasered outside the Houses of Parliament on Thursday amid claims he may have been armed with a knife.
Pictures on social media show the man being restrained by armed officers shortly before 11.30am.
"There were shouts of 'knife, knife, knife' and man was brought down with a Taser," Evening Standard reporter Joe Murphy wrote on Twitter.
The Metropolitan Police said the man, aged in his 30s, was arrested on suspicion of possession of a knife.
It said no one was injured during the incident.