All At Once
Adverb
いっせいに
派遣社員たちは、景気の悪化で一斉に解雇されることを心配しています。
The temporary workers are worried about being fired simultaneously in the recession.
一斉 is simultaneous or all at once. Add the に in there and you're heading towards those things. For "simultaneous" that becomes simultaneously. For "all at once" it stays at all at once.
The reading comes from 一斉.