Roy Keane believes Manchester United fans have "had enough" and predicted further protests after Sunday's game with Liverpool was postponed due to scenes at Old Trafford and in the city.
Ole Gunnar Solskjaer's side were due to face the reigning Premier League champions at home at 16.30 local time, but kick-off was delayed due to demonstrations against the club's owners, the Glazer family.
After protestors assembled outside the ground, some gained access to Old Trafford and entered the pitch, while others gathered outside the Lowry Hotel where the United squad stay prior to home matches.
Around an hour after the game had been due to kick off, United announced it had been postponed "due to safety and security considerations around the protest". The clubs will agree to a new date with the Premier League.
United fans have long been vehemently opposed to the Glazer ownership due to the significant levels of debt added to the club following their leveraged takeover in 2005, but the failed attempt by the Red Devils and the Premier League's 'big six' to join a breakaway European Super League has fuelled animosity in recent weeks.
While the Super League appears dead in the water for now after the six English clubs withdrew from the plans, anger towards executives such as the Glazers and Arsenal owner Stan Kroenke has abated little among supporters.
Former United captain Keane believes Sunday's protests were just the start, telling Sky Sports: "The United fans have had enough and they're doing it because they love the club.
"It's not just as a result of what's happened the last two weeks with the Super League... it's been building for a number of years. They've come to the end and feel enough is enough. It's a huge statement for the game to be called off.
"There's been a build-up in tension, whether it be about ticketing, poor communication, things going on in the background.
"The leadership of the club has not been good enough. When they look at the owners, they feel it's just about making money. The United fans have looked at the Glazers and thought 'enough is enough'.
"They're doing it because they love the club. Some people won't agree with it but sometimes you have to put a marker down for people to take notice.
"This will go out all over the world, and hopefully the owners of Manchester United will sit up and take note. These fans are deadly serious and there's more to come, and this is just the start of it from United fans – I guarantee it."
Fellow former captain Gary Neville, who was scathing over the proposed formation of the Super League, said: "I think it's a warning to the owners of the football club that, ultimately, they're not going to accept what they've done in the last couple of weeks.
"Beyond today, it should be about reform. Protesting is the right of every single person in this country, we must retain that, but beyond today, it's about making sure that fans across the country unite to make sure there's reform in English football, or else [these protests] will be a waste of time.
"There are people protesting in London today against the government trying to stop protests! We have to allow protests, but there is always the potential of there being a flashpoint because there will always be one or two people who may have had a drink, they might just be coming to antagonise, or they might be coming for ulterior motives. That will always happen.
"The reality is that what happened two weeks ago has ignited the United fan base into life again. It's brought them back to life.
"Fifteen years ago, there were a lot of fans who were upset, and every one I speak to are absolutely disgusted by what happened a fortnight ago. United were leading this thing, as were Liverpool, two weeks ago. They are the biggest clubs in this country."