London's weekend marathon will be a big event.
More than 36,000 runners are signed on to pound the pavement. This year the pack will include double Olympic gold medalist Mo Farah, as well as Kenyan runners Wilson Kipsang and Patrick Makau.
Unlike the marathon in Boston, which starts out well outside the city, London's course loops and weaves through the very heart of the city.
For police, it's a security nightmare.
Retired police commander Bob Broadhurst was in charge of the police operation for London's 2012 Olympic and paralympic games.
He says officers in London will do everything they can to stop the type of assault that happened in Boston.
"But of course you can't legislate for a loan individual when you've got crowds of several thousands on the streets. You just can't search everything," Broadhurst says. "The price you pay in a free society for allowing free movement is that you are vulnerable to the kind of heinous people that want to do the kind of attack that they've done in Boston."