En masse is an adverb used to describe a group doing an action all at once or as a single entity. It is an appositive, which means it comes after the word it modifies. People run en masse. It comes from French and literally means in a mass.
Sometimes en masse is used as an adjective. The dictionary does not define this as an officially accepted use, but it is common enough that the meaning should be clear to readers. Convention seems to dictate that when en masse is an adjective it loses the appositive quality and comes before the noun it modifies.
It may be pronounced (in mas) or (awn mas), though the second is only listed in American dictionaries and is most likely the result of perpetual mispronunciation. When pronounced (in mas) it also leads to frequent spelling errors, especially if the user is unaware the phrase came from French.
The Madras High Court has set aside en masse termination of the staff recruited in the Indian Maritime University here and directed the university to continue the services of those who had approached the court and were working as on date. [The Hindu]
Civilians were largely spared in the fighting because they evacuated en masse, mostly across the border into Turkey, in the early stages of the battle. [Yahoo News Australia]
The call to defy the ban that spread through the social networking sites was the largest en masse action since November 1990, when a group of 47 Saudi women were arrested and severely punished after demonstrating in cars. [The Telegraph]