Euston Road is closed both ways between Eversholt Street and Great Portland Street. @TfLBusAlerts describes a "police incident" and there are photos on Twitter of a bomb disposal robot heading into the Euston underpass, as well as reports of local office workers being told to stay away from the windows.
Mark Walley in the comments says the suspicious package is being described as an incendiary device, rather than an actual bomb.
Update 3.25pm: It's looking like Johnny Five has done his work and the situation is resolved. Confirmation that Euston Road is reopen comes from TfL's traffic news Twitter account. Honestly, what with this and the trains, let's hope transport disruptions don't come in threes today.
Photo by Dominic Robinson via Twitter