Everyone was packed and ready to leave at a moment's notice.
Related vocabulary: at the drop of a hat, on the spur of the moment
at a moment's notice
With little time to prepare; immediately and at any time. Firefighters need to be able to spring into action at a moment's notice.I can't give a speech at a moment's notice!
at a moment's notice
and on a moment's notice
with very little advance notice; with just a little bit of warning. They are always asking us to produce reports at a moment's notice.