Look up the largest stadium in the world, Strahov in Prague, that hosted the Communist “Spartakiad” parade every May 1st. Almost every performer and attendee arrived via the high-capacity tram and bus terminal Smyčka Dlabačov, often after taking a train and the metro. As a result, the parking lot is absolutely tiny.
Well, I’m guessing they concealed them in an extra-large Li-Ion battery and wired a heating element in series inside it, so that shorting the terminals by the circuitry triggered the explosive. Pagers use so little power that the lower capacity would be hard to notice and the heating element’s voltage drop would be negligible. I assume the pagers’ command & control equipment had backdoors, too.