Manchester United fans are furious that US tycoon Malcolm Glazer has launched a formal takeover bid for the club.
Glazer has a 70% share after securing the 28.7% stake of Irish racing tycoons JP McManus and John Magnier.
Nick Towle, chairman of fans' pressure group, Shareholders United, claimed he has ripped up his season ticket.
He said: "I am very sad - I still love the club but I refuse to put a penny into the company. And I believe as many as 20,000 fans may also leave Man Utd."
Around 2,000 fans gathered at Old Trafford on Thursday evening after hearing the news, and some marched around the ground with a huge "Not for sale" banner.
There was also a "sit-down" protest with fans blocking a main road next to Old Trafford - while others burned effigies of Glazer and season-ticket renewal forms in a gesture of defiance.
Five people were arrested for public order offences, with three men charged with the offences.
A spokesman for Greater Manchester Police said: "There have been some arrests for public order offences.
"The protest has been fairly peaceful, but the group were warned under the Public Order Act to move on and clear the site."
Police were also called to the Manchester Art Gallery after reports protestors entered a private function, but no arrests were made.
Supporters are concerned about the level of debt involved in Glazer's bid and fear for the club's future.
Oliver Houston, vice chairman of Shareholders United, told BBC Radio Five Live that Glazer is "no Roman Abramovich".
Chelsea's Russian billionaire owner, Abramovich, has invested over £200m on players since taking over, bankrolling the club to the Premiership title this season.
Houston predicted an entirely different scenario at United.
He said: "He's not turning up with a suitcase full of his own cash.
"He is, in effect, asking Manchester United fans to pay for his takeover, to pay for increased ticket prices and increased merchandising."
Shareholders United, which represents 17% of the club's stockholders, still hopes to prevent Glazer reaching the 75% level of shares he needs to make his offer unconditional - and essentially own the club.
But Towle admitted: "It's looking like an uphill battle for us."
Fellow member of Shareholders United, journalist Michael Crick - biographer of United manager Sir Alex Ferguson - believes Glazer's "hard-nosed business record" spells danger.
He said: "He is going to be much keener to boost profits, particularly as he's going to have to borrow so much to buy the club.
"I can only see that it's the fans who are going to lose out here in a big way."
MP Tony Lloyd, who represents Manchester Central and is also a Manchester United shareholder, told Five Live:
"I'm a very modest shareholder, I wish I had more so that I could do my best to block what I think is an unwelcome bid.
Fans burned season ticket renewal forms as an act of defiance
"I think most football supporters in this country will see this as being an undesirable move because it's not in the interests of the game of football."
Sports minister Richard Caborn added: "Whoever owns Manchester United is a commercial decision, but anyone seeking to buy the club would be foolish not to take account of the views of the fans.
"I hope Mr Glazer sets up constructive discussions with the club, its supporters and employees, as well as the footballing authorities, as soon as possible.
"It is important that the club continues to play its central role in English football in the years to come."
However, Jules Spencer, chairman of the Independent Manchester United Supporters Association is ready to take radical action.
"If this is to be the end then we will go down kicking and screaming," he said.
"If Glazer does get control, then we will up sticks and form a new club which will continue the traditions and heritage and the legacy of 125 years of Manchester United.
"We don't want Malcolm Glazer, we want a club run for the fans - and that is what we will do."
And former Red Devils manager Tommy Docherty added: "Manchester United's heart and soul has been sold today - it has changed for ever."