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.
READ MORE: Cardiff City v Millwall Live
Dozens reported outside the club's main reception at Cardiff City Stadium, a couple armed with banners which read 'Tan Out', before bursting into a number of anti-owner and anti-board chants.
Chants which could be heard were "We want our club back," "Sack the board", "We want Tan out" and "I'm City 'til I die".
As the chants reached a crescendo, more fans entered the arena and one even lit a blue flare before the chorus of supporters burst into a rendition of "We're Cardiff City, we'll always be blue".
You can see the protests in the photos and videos below
The songs continued into the Millwall match, too, with "Vincent Tan, get out of our club" sung at the start of the match from the Canton End.
Tan has been the owner of Cardiff for more than 12 years. He has overseen two promotions to the Premier League, both of which resulted in immediate relegation back down to the Championship. It is estimated he has put more than £200m of his own personal wealth into the club.
He has also courted controversy, however, changing the kit from blue to red and also changing the badge. Financially onerous court cases, including that of Emiliano Sala's transfer from Nantes, have detracted attention away from the football in recent years.
As a result, less money has been made available to recruit players and managers and that has gone hand in hand with the club's perennial decline down the table.
Cardiff have had four managers in two years and are on the lookout to appoint a fifth, with Dean Whitehead currently the interim boss.
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