Her Majesty's Royal Guards
Royal Guards #1 
     | Royal Guards #2 
     | Royal Guards #3 
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As you walk from Trafalgar Square to Westminster, the parade ground where the “Changing of the Guard” takes place is on the right-hand side of the road. There are always some royal guards on duty, up to 7pm when the gates are closed. Tourists, including what I think are US Naval Officers in the first picture above, gather to stand, stare and have their photograph taken. The Royal Guards are trained not to care; they just go about their business as if nobody else is there.
You can view some more Royal Guard photographs here: Royal Guards Album