Police cordoned off Whitehall, which is home to several government departments including the foreign office and defence ministry and leads to Johnson’s Downing Street office and residence, for several hours but it was later reopened.

An investigation was launched following the incident at Horse Guards Parade, a ceremonial parade ground which overlooks St James’s Park, police said.

“At around 8.50am, a 29-year-old man, who was armed with a knife, confronted two defence ministry police officers. Taser was deployed and the man was restrained by officers. There were no reports of any injuries,” London’s Metropolitan Police said in a statement.

“He was arrested on suspicion of attempted murder and possession of an offensive weapon and remains in custody at a central London police station. The incident is not being treated as terror-related.”