A second man has been arrested after three police officers were injured by a man brandishing a 4ft sword near Buckingham Palace on Friday.
A 30-year-old man has been arrested on suspicion of terror offences, and a search of a property in west London is being carried out.
It comes after a man drove at police outside the Queen’s official residence.
A 26-year-old arrested at the scene on Friday evening remains in custody.
Police have been given more time to question him – the warrant for his further detention will expire on 1 September.
Dean Haydon, head of counter-terrorism at Scotland Yard, said a man deliberately drove at a police vehicle before stopping and being confronted by three officers.
He repeatedly shouted “Allahu Akbar” as CS gas was used to “quickly” restrain him, Cdr Haydon said.
The three unarmed Westminster police officers received minor injuries.
Two of them were taken to hospital after receiving cuts to their hands and forearms.
No members of the Royal Family were in Buckingham Palace, which is the Queen’s official London residence, at the time.