It is currently free to climb the 334 steps of the clock tower containing Big Ben.

BBC: U-turn over Big Ben trip charges

Police will surely send extra resources to those parts of the course near historical sites such as Tower Bridge and Big Ben the kind of landmarks that attract disproportionate attention from those who seek to create violent spectacles of mayhem.

WSJ: William Bratton: A Cop's-Eye View of Terrorism

As a mark of respect, the chimes of Big Ben, as the bell and clock tower by the Palace of Westminster are commonly known, were silenced for the duration of the proceedings.

CNN: SHARE THIS

The London Marathon route, which is lined by hundreds of thousands of spectators every year, starts in Blackheath and finishes near Buckingham Palace, passing some of the capital's most recognisable landmarks including Tower Bridge, Canary Wharf and Big Ben.

BBC: London Marathon runners