Atletico under scrutiny after fan invades field
The incident occurred after the final whistle of the UEFA Champions League quarter-final first leg between Atletico and Real Madrid at the Vicente Calderon.
A member of the crowd made it to the side of the pitch and grabbed the shirt of Ronaldo, who embraced the man before security guards led the intruder away.
A UEFA statement on Wednesday confirmed the opening of the proceedings, as well as citing a charge of "insufficient organisation" for stairways being blocked.
The case is due to be dealt with on May 21.