The heavily armed swooped on the scene moments after multiple shots were fired outside Westminster station and Portcullis House.
Specially trained officers were spotted patrolling the perimeter fence of the Houses of Parliament while MPs were locked-down inside the Commons.
Reports of two incidents have emerged after a car ploughed through a crowd of people on Westminster Bridge shortly after 2.30pm this afternoon.
Twelve people were reportedly injured before the car continued towards Parliament, causing people to flee and jump into the Thames to avoid being struck.
Highly trained terror police react to the London terror attack in Westminster
At least one woman died in the incident on the bridge due to “catastrophic injuries”, with Sky News reporting a second person was killed.
The car then reached Parliament where one knifeman is though to have got out of the car and attacked a police officer before being shot dead.
Theresa May was bundled into a police car by plain-clothed officers and evacuated.
MPs were debating in the Commons at the time of the attack and have been prevented from leaving the chamber until the security services give them the go-ahead to leave.
The specialist units seen responding after the deadly attack are kitted out in grey Kevlar body armour and carry a cache of weapons including sniper rifles, automatic assault rifles, handguns, submachine guns and tasers.
They were introduced to London after a spate of deadly terror attacks across major European cities.
Days before the attack armed police played out the chaos of a “pleasure boat” terrorist attack on the Thames in a unique marine-based exercise to test the emergency response to gunmen in London.
Get Quotes on Home Insurance
London Terror Attack: LIVE PICTURES Wed, March 22, 2017
Two dead, 12 injured as knifeman mows into crowd in Westminster
PA 1 of 36
Emergency services at the scene outside the Palace of Westminster, London, after policeman has been stabbed and his apparent attacker shot by officers in a major security incident at the Houses of Parliament
A sightseeing vessel became the scene of a fierce mock-gun battle between armed officers and police volunteers posing as terrorists shortly after 11am on Sunday, close to the London Docklands area.
The Exercise marks the first time the Metropolitan Police have enacted a live-action, water-borne exercise of its kind, with over 200 officers involved.
Sky News reported the highly-prepared units arrived on the scene within six minutes of the attack on Westminster.
A spokesman for the Met Police told reporters they will maintain a heavy police presence around London to ensure the safety of the public and urged anyone with evidence or information to contact them on 0800 789 321.