News of his operation was made public by his old club, with a United statement cautiously optimistic about the prospects for the 76-year-old.
PIC: Ferguson retired five years ago having won a 38 major trophies in just over 26 years in charge at Old Trafford. (AFP)
Messages of support poured in for Alex Ferguson after the legendary former Manchester United manager was left fighting for his life following emergency surgery for a brain haemorrhage.
News of his operation was made public by his old club, with a United statement cautiously optimistic about the prospects for the 76-year-old, the most successful manager in British football history.
"Sir Alex Ferguson has undergone emergency surgery today for a brain haemorrhage," the statement said.
"The procedure has gone very well but he needs a period of intensive care to optimise his recovery. His family request privacy in this matter.
"We will keep Sir Alex and his loved ones in our thoughts during this time, and we are united in our wish to see him make a comfortable, speedy recovery."
An ambulance was called to the Scot's home in Cheadle, near Manchester, in northwest England at 9:00 am (0800 GMT) on Saturday, reported the Daily Mail.
Concerns for Ferguson's health started to mount when it was announced his son, Darren, the manager of English third-tier club Doncaster, would miss his side's match against Wigan because of "family reasons".
'Keep fighting Boss'
"It is tragic. I am going to find out how he is as soon as I possibly can," said Everton manager Sam Allardyce, a longstanding friend of Ferguson's, after his side's 1-1 Premier League draw at home to Southampton.
"Obviously I knew it was extremely serious when I knew his son Darren didn't attend his own game," the former England boss added. "I hope he has a full recovery."
Southampton manager Mark Hughes, a United striker during Ferguson's early years at Old Trafford, said: "I only just got a little whisper earlier on. I was hoping it wasn't true. I wish him all the best."
David Beckham was among several United players past and present who took to social media to offer their support to "the boss".
On Instagram, with a picture of him as a youngster alongside Ferguson, Beckham wrote: "Keep fighting Boss.. Sending prayers and love to Cathy and the whole family x @manchesterunited."
Real Madrid superstar Cristiano Ronaldo, signed by Ferguson as an 18-year-old in 2003, tweeted a picture with his former mentor. "My thoughts and prayers are with you, my dear friend. Be strong, Boss!" he said.
Former United and England captain Wayne Rooney said on Twitter: "Get well soon Boss. Thoughts with all the family at this sad time. #AlexFerguson."
There was a heartfelt message from former United goalkeeper Edwin van der Sar and his wife Annemarie, who herself recovered from a brain haemorrhage in 2009.
"Devastated about the news about Sir Alex and knowing all too well about the situation ourselves. Stay strong and hope together with everyone you recover. Edwin & Annemarie," said the couple's Twitter post.
Another former United goalkeeper Peter Schmeichel tweeted simply: "Please. Be strong. Win this one" interspersed with prayer emojis.
Midfielder Michael Carrick said he was "absolutely devastated", adding, "All my thoughts and prayers are with him and his family. Be strong Boss."
Vincent Kompany, the captain of new English champions Manchester City, summed up hundreds of messages from rival clubs and from right across the football spectrum when he said: "Hang in there Sir Alex. Thoughts are with the family and close ones."
Away from football, Nicola Sturgeon, the First Minister of Scotland, tweeted: "My thoughts are with Alex Ferguson and his family - wishing him a full and speedy recovery."
Ferguson retired five years ago having won a 38 major trophies in just over 26 years in charge at Old Trafford, including 13 Premier League titles and the UEFA Champions League twice.
He was at United's home ground a week ago to present a commemorative vase to old rival Arsene Wenger, who is stepping down as Arsenal manager.
Ferguson had planned to retire at the end of the 2001/02 campaign but changed his mind after speaking to his wife Cathy and their family.
But the death of his wife's twin sister in 2012 provided Ferguson with what he later wrote was a "watershed moment.
"I felt that, after all those years during which Cathy had put me first, it was time that I took care of her needs," he explained.
Prior to arriving at Old Trafford, Ferguson, enjoyed success as manager of Aberdeen, interrupting Glasgow giants Celtic and Rangers' Old Firm dominance of Scottish football by winning three league championships, four domestic Cups and the 1983 European Cup Winners' Cup in charge of the Dons.
It was a similar story at Old Trafford, where his United teams ended Liverpool's dominance of the domestic game, with the highlight being the 1999 treble of Premier League, Champions League and FA Cup.