The London Marathon poses a major security headache for police barely a month since terror struck the heart of the capital. Some 750,000 spectators will cheer on the race, which snakes for 26.2 miles along both sides of the Thames, presenting a tempting target for extremists.
Armed police will be scrambled to different points along the course to keep watch, and lines of police vans were visible along The Mall on Sunday. Vehicles have time and again been the weapon of choice for attackers in recent European atrocities and will be a focus for race day security efforts.
On March 22, Khalid Masood showed the deadly potential of the tactic when he mowed down pedestrians on Westminster Bridge, killing four people. He was the latest in a murderous roll call of terrorists who have targeted crowds on the continent, including similar assaults on Nice, Berlin and Stockholm in the last year.
By Sunday morning, steel barriers had been pulled out into the roads along key points of the route, including the mouth of the final stretch leading down to Buckingham Palace. The road blocks are designed to stop vehicles and were put in place especially for the event, police said.
Memories of the attack on the Boston Marathon in 2013, when a bomb killed three people near the finishing line, will also be at the front of the minds for security officials. Hundreds of officers from the Metropolitan, City of London and British Transport Police will be involved in protecting the public from harm.
The Met claimed in the run-up to race day they had no specific information about an attack on the event.