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Security measures around Westminster and on central London bridges were beefed up in the wake of terror attacks last year.
Within days of the atrocity at London Bridge and Borough Market in June 2017, barriers were put in place on bridges including Westminster, Waterloo and Lambeth in a bid to stop terrorists mowing down pedestrians.
Tactics for armed officers were also changed, with marksmen allowed to shoot at a vehicle being used in such an attack.
Previously, firearms officers had the option of shooting at a moving vehicle, but this was discouraged as it could increase the risk to the public.
But the approach was revised so that firing at a car, van or lorry when it is on the move is an accepted tactic for such incidents.
There are also more armed patrols on the streets at any one time, with the number of firearms officers and vehicles having gradually increased since 2016.
Police trained to shoot terrorist drivers under new rules
A £143 million plan to boost armed policing was announced in the months after the terrorist attacks in Paris in November 2015, in which 130 people died and hundreds more were injured.
Security arrangements around Parliament are likely to come under scrutiny at the forthcoming inquests into the deaths of five people including police officer Keith Palmer who were killed in the Westminster Bridge attack last year.
Khalid Masood ploughed a hired SUV into pedestrians on the bridge before getting out and fatally stabbing Mr Palmer, who was guarding the Palace of Westminster but was unarmed.