一斉いっせい Common Adverb Noun English jmdict 1 adverb (fukushi)noun (common) (futsuumeishi) simultaneous;all at once
一斉にいっせいに Common Adverb English jmdict 1 adverb (fukushi) simultaneously;all at once;in unison 人々が一斉に走り出した。 People started to run all at once.
Shirabe