A ring of steel and concrete barriers has been erected along the route of Sunday's London Marathon to protect it from the threat of a possible terrorist attack.
‘Hostile Vehicle Mitigation Barriers’ designed to stop a hijacked car or lorry being driven at the crowds were put in place overnight.
At the same time extra armed officers will patrol the area along the 26 mile route, with police spotters sighted on buildings overlooking the event.
The increase in security comes in the wake of the Westminster attack, which saw five people killed after Khalid Masood ploughed a hire car into pedestrians before being shot dead outside Parliament.
The shooting of a police officer on the Champs Elysees in Paris on Thursday, along with death of four people when a hijacked lorry was ploughed into a Stockholm department store earlier this month have also raised fears about further attacks.
It emerged last week that police have been instructed to shoot terrorists at the wheel of a vehicle to stop an attack and have been issued with ammunition to penetrate doors and windows.
Officers had previously been told not to shoot drivers because of the additional dangers it posed, but the advice was changed in the wake of recent vehicle attacks in Europe.
While police emphasise there has been no specific threat made against the London Marathon, Scotland Yard pointed out that the threat level in the UK remains at severe, meaning an attack is “highly likely”.
A Scotland Yard source said: “There will be firearms officers deployed as part of the operation to police the marathon. This is over and above the usual firearms response capability that we would normally have available across London on a typical day.”
The thousands of people flocking to central London to watch the event have been urged to remain vigilant and report any suspicious behaviour.
Commander Jane Connors, who is leading the policing operation, said: “Our priority is to ensure that those taking part and watching have a safe and enjoyable experience and I’d like to reassure people that there is a comprehensive policing plan in place to achieve this."
She added: “There will be a strong, visible presence of officers along the entire route and at transport hubs across London, but if you notice anything suspicious, then please report it to us.
“I’d also like to emphasise that given the nature of the event, it is very rare to see crime or disorder, but I’d ask anyone planning to attend to take basic precautions to keep their possessions safe.”
Hundreds of officers from the Metropolitan Police, City of London Police and British Transport Police will also be on duty throughout the day.
Each of the 50,000 runners taking part have been emailed advising them to meet their families and friends away from finish line at The Mall, where security will be at its tightest.
Virgin Money London Marathon, the event's organisers, said they were working closely with the Met police.