Two security guards at MI5's headquarters in London have been attacked by a man carrying a machete.
The attacker's black boots came off during his arrest
The man is thought to have entered Thames House, in Millbank, through the Northern Ireland Office door and left through the main MI5 entrance.
He was subdued by armed police with a stun gun after the attack at about 1315 BST on Friday. A man has been arrested and is being questioned.
The guards' injuries are not thought to be life-threatening.
Taser stun gun
One of them received cuts to the hand and the other suffered cuts to the arm.
The attacker was also taken to hospital and later discharged.
He was thought to have been carrying a green rucksack which was lying on the steps near the front door of the building.
His black boots, which came off when police seized him, were found left lying nearby on the pavement.
He was hit twice with a Taser stun gun after a police armed response unit arrived.
Following the attack a dozen police vehicles and motorcycles were parked outside the building.
Millbank was closed between Lambeth Bridge and Thorney Street but has re-opened.
A Scotland Yard spokesman said: "At 1.15pm we were called to Thames House, Millbank, to reports of an intruder armed with a knife.
"We discovered a man outside the building armed with a machete and a large knife. He was challenged by officers and refused to co-operate."
He said it was believed the man then entered the building and attacked two security guards.
"A Taser was discharged by police and the man was arrested and taken by an ambulance to hospital," he added.
A man, who is white and thought to be aged about 40, has been arrested on suspicion of burglary and assault.
He is being questioned by police at Belgravia police station.
MI5 is the UK's security service, responsible for protecting the UK against threats to national security.