Simultaneously, All At Once
adverb
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.
いっせいに
The reading comes from 一斉.
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.