Police warned people to avoid Parliament Square; Whitehall; Westminster Bridge; Lambeth Bridge; Victoria Street up to the junction with Broadway and the Victoria Embankment up to Embankment tube.
Westminster station, which faces parliament, was also been evacuated.
Armed officers patrolled the streets after the incident and an announcement in parliament revealed a suspicious package was found in a vehicle.
Bomb squad was drafted in to to assess the fresh threat.
The incident has been called a terror attack by the Met Police,
— Met Contact Centre (@MetCC) March 22, 2017
Just before 3pm on Wednesday multiple people were injured after a car ploughed into people on Westminster Bridge, before continuing its path of destruction towards Parliament.
The driver is said to have made their way into the Old Palace Yard before stabbing an officer, before being shot by armed police.
Five people, including one woman, have been confirmed dead and more are said to have suffered “catastrophic injuries”.
Among the dead are a police officer, named as Keith Palmer, and the as-yet unnamed terrorist.
At least 40 people were injured.
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The London Ambulance Service (LAS) has urged anyone who does not critically need an ambulance to refrain from dialling 999.
A statement from them said: “We are working closely with other members of the emergency services at the scene, with our priority being to ensure patients receive the medical help they need as quickly as possible.
Armed officers are patrolling the streets after the incident
Only call us in a genuine emergency
London Ambulance Service
“We have declared a major incident and our priority is to assess patients and ensure that they are treated and taken to hospital as soon as possible.
“As we are very busy dealing with this incident, we would ask the public to only call us in a genuine emergency.”
The Met has also reminded people not to dial 999 unless an emergency.
- Witness reports of knife attack on police outside Parliament
- Police rush to scenes at Parliament
- Smoke comes from car that ploughed into Houses of Parliament
London Terror Attack: All the latest pictures Thu, March 23, 2017
5 dead and 40 injured in Westminster terror attack
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Conservative MP Tobias Ellwood (centre) helps emergency services attend to a police officer outside the Palace of Westminster, London
They tweeted: “Our lines are busy, please dial 999 for emergencies only.”
The Met is appealing for anyone with footage to get in touch on www.ukpoliceimageappeal.co.uk.