WordNet (r) 3.0 (2006):

all at once
    adv 1: all at the same time; "Let's say `Yes!' all at once"
           [syn: all together, all at once]
    2: without warning; "all at once, he started shouting" [syn:
       all of a sudden, all at once]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "all at once":
   abruptly, all together, at a blow, at a stroke, at once,
   at one blow, at one jump, at one stroke, at one swoop, at one time,
   bang, collectively, communally, conjointly, corporately, dash,
   en masse, ensemble, forthwith, hastily, impetuously, impulsively,
   in a body, in a hurry, in association, in company, in conjunction,
   jointly, like a flash, like a thunderbolt, mutually, now,
   of a sudden, on short notice, per saltum, plop, plump, plunk, pop,
   precipitantly, precipitately, precipitously, pronto, right away,
   right now, right off, sharp, simultaneously, slap, smack,
   startlingly, straightaway, straightway, subito, sudden, suddenly,
   surprisingly, then and there, this minute, this very minute,
   together, unawares, unexpectedly, unitedly, uno saltu,
   without delay, without notice, without warning