Bradford Bulls say they have been inundated with pledges after revealing they need £1m to stay in business, with at least half needed within 10 days.
The club urged fans to pledge £100 each to help clear debts, as players were told they may not be paid in April.
Bradford say an impending tax bill and a changed banking lending arrangement has left them with virtually no cash.
Ex-Bulls stars Jamie Peacock and Stuart Fielden pledged support, along with the Lord Mayor of the City of London.
The four-time Super League champions sold the lease on their Odsal ground to the Rugby Football League in January to raise funds, with the RFL renting the ground back to the Bulls.
A statement on the club's website read: "The RFL stadium deal only enabled us to address our long-term liabilities but could not help us stave off the grave financial situation."
Bulls chief executive Ryan Duckett told BBC Radio Leeds a changed lending arrangement by the Royal Bank of Scotland (RBS) on the stadium deal had "brought things to a head".
RBS said it had not cut the Bulls' overdraft, but that the directors had failed to provide alternative security to secure credit since selling the lease on Odsal.
England captain and Leeds Rhinos prop forward Peacock, who made over 200 appearances for the club, said on Twitter he would pledge £800 to the cause.
While Fielden, whose sale to Wigan in 2006 raised a world transfer record £440,000 for the Bulls, offered to help out by manning the phones at the club during his rehabilitation from injury.
Fan Ralph Scott also stepped forward to pledge £5,000 before the deadline to raise funds at 1700 BST on 6 April.
Duckett continued: "It's a serious situation and it's taken a lot of people by surprise, including staff and players.
"But in the four or five hours since the news broke, the response has been fantastic, the phones have been red hot.
"Someone has just pledged £5,000 and we've had ex-players also pledging their support,
"It is those things that will help build the momentum.
"The passion people have showed so far and some of numbers pledged make me feel very confident we're going to get through it."
Earlier he said he felt the club had to be "pro-active to address some of the issues we've got rather than letting them escalate, and that's why we've gone out with this radical message".
He added that if fans did not get behind it "there might not be a Bradford Bulls".
Bradford-born Lord Mayor of the City of London David Wootton has also backed the campaign, calling "on all fans of rugby league to help support this great institution overcome its current problems".
Earlier, chairman Peter Hood told the Bradford Telegraph & Argus: "If we haven't got cash then we can't stay alive and if we can't stay alive then we can't fulfil our fixtures. It's that serious."
Bradford won their last title in 2005 and have also won five Challenge Cups, but a lack of recent on-field success has compounded their financial problems.
RFL director of standards and licensing Blake Solly told BBC Radio Leeds: "We're concerned as the governing body of the sport, but we're giving them all the help we can.
"We're working with them on a day-to-day basis to see how the pledge scheme is going and trying to give them all the logistical support possible."
Fans spokesman Mike Farren admitted the news about the club's precarious financial position had come as a shock to the supporters.
"We've always worked closely with the club, but I was unaware of how serious the situation was. This was a bombshell," he said.
"We are certainly concerned, but very much hope the club can pull through. But, to raise £500,000 in 10 days seems extremely ambitious."
Figures from other Super League clubs have rallied support, with Leeds Rhinos half-back Rob Burrow using Twitter to urge supporters of rival teams to boost attendances at Odsal.
West Yorkshire neighbours Castleford released a statement from chief executive Richard Wright that offered his "best wishes in their bid to survive".
"It would be a tragedy for the game to lose any professional club, never mind one of such tradition and history."
St Helens interim coach Mike Rush added: "It's sad that any club is struggling financially, hopefully the fans can rally round."