Synonyms of at once in English:

at once

See definition of at once


1‘you must leave at once’

immediately, right away, right now, this instant, this minute, this moment, this second, now, straight away, instantly, instantaneously, directly, suddenly, abruptly, summarily, forthwith, promptly, without delay, without hesitation, without further ado
quickly, as fast as possible, as soon as possible, fast, speedily, with all speed
informal like a shot, in a flash, like a flash, before you can say Jack Robinson, in two shakes, in two shakes of a lamb's tail

later, in due course

2‘all the guests arrived at once’

at the same time, at one and the same time, at the same instant, at the same moment, together, all together, simultaneously
as a group, in unison, in concert, in chorus

singly, in dribs and drabs