Witnesses have described an “almighty smash” and officers with Tasers and “rifles” shouting at a man after a car drove into the Downing Street gates.
One person was arrested after a hatchback hit the barrier at around 4.20pm.
Video showed it crossing Whitehall, apparently from the direction of a Ministry of Defence car park opposite, but then appearing to slow down as it approached.
Police said a man had been detained on suspicion of criminal damage and dangerous driving.
No one was hurt.
Witness Simon Parry, 44, said: “I heard a bang and looked up and saw loads of police with Taser guns shouting at the man.
“A lot of police vehicles came very quickly and were very quick to evacuate the area.”
He said the man had his “face to the floor” and that “there were sniffer dogs and a bomb squad”.
Mr Parry added: “We saw people that were in a panic running away and we saw people who were excited.”
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Witness: There was an ‘almighty bang’
Another witness, Krish Kandiah, who was entering Downing Street at the time, said he heard an “almighty smash at the front gate, which was metres away from us”.
“Immediately we saw police scramble and armed officers responded swiftly and shouted at the man to get out the car,” he said.
Mr Kandiah – who was handing in a petition – estimated there were 10 armed officers with “rifles” around the man.
“He was wrestled out of the car very quickly and put down on the ground and handcuffed and his vehicle was then searched,” he said.
He praised police, who pushed his group through security and formed a “human shield” to protect them.
The road outside Downing Street was cordoned off after the incident as police searched the car – which had its boot open.
Sky News correspondent Joe Pike, at the scene, said officers seemed calm and “not on edge at all”.