More than 100 Cardiff City fans gathered outside the club's reception to protest against owner Vincent Tan and the board.
There had been grumblings of discontent after the sacking of Mark Hudson last week and the movement gathered pace on social media after the Bluebirds' chastening 5-2 defeat at the hands of Leeds United in the FA Cup this week.
Fan accounts on social media called for a protest before the Millwall match to make their grievances known to the club's hierarchy.
