From ABC News:

Every year the Boston Marathon attracts thousands of spectators who cheer on runners, but this year thousands more eyes – some digital and some undercover – will be watching intently as well, looking for anything that might warn them of a repeat of last year’s tragedy. Authorities in charge of security for the marathon Monday are taking no chances, with an unprecedented security apparatus that includes some 4,000 deployed police officers, including 500 plainclothes officers, more than 100 surveillance cameras and a underground, futuristic coordination center boasting some 260 security officials representing more than 60 local, state and federal agencies.