at once

1)

you must leave at once

Syn:

immediately, right away, right now, this instant, this second, this minute, this moment, now, straightaway, instantly, directly, forthwith, promptly, without delay, without hesitation, without further ado; quickly, as fast as possible, as soon as possible, ASAP, speedily; informal like a shot, in a flash, pronto, in two shakes (of a lamb's tail)

Ant:

later, in due course

2)

all the guests arrived at once

Syn:

at the same time, at one and the same time, (all) together, simultaneously; as a group, in unison, in concert, in chorus

Ant:

singly, in dribs and drabs