Coach Georgy Yartsev threw out Mostovoi after he blamed the team's 1-0 defeat by Spain on Saturday on a strenuous training schedule and said Russia was unlikely to qualify from the group stage of the competition.
"Mostovoi has been expelled because of comments he made in the Spanish press," a football federation official said.
Valery Draganov, vice-president of the Russian Football Union, said the decision to fire Mostovoi was hasty.
"I really regret this decision, it does not help the team," Draganov told Ekho Moskvy radio.
"Even if Mostovoi spoke out critically against the coach, I don't think it is a reason for such a quick dismissal. We don't have enough players of Mostovoi's calibre -- people like that are worth their weight in gold."
Russia face hosts Portugal on Wednesday in their second Group A match.
