"Thanks for everything, Boss." Cristiano Ronaldo on Twitter. (Note: Adjoining image courtesy Ronaldo's official Twitter account)
"2001-2006, 219 games,150 goals under the most successful manager in football history. It was a unique privilege." Ruud van Nistelrooy on Twitter
"His achievements in the game place him without doubt as one of the 'greats' ... Will his longevity at the top ever be repeated?" - FIFA President Sepp Blatter.
"No one has made as great a contribution to the Premier League than Sir Alex Ferguson. His drive, ambition, skill, passion and vision have not only shaped Manchester United, but in many ways the game of football as we now know it." - Premier League chief executive Richard Scudamore.
"His CV is almost unique in a results-based profession that normally focuses on short-term solutions rather than long-term vision. He is a true visionary." - UEFA President Michel Platini.
"Sir Alex Ferguson's achievements are truly remarkable - he is genuinely one of the greatest managers of all time and certainly of the modern era." - English Football Association chairman David Bernstein.
"An end of an era today! Sir Alex Ferguson, the greatest of all time! United will have a tough time trying to replace him!" - Golfer Rory McIlroy.
"It's just not sinking in! Manchester United with no Sir Alex just doesn't feel right. What a man and great that it has ended on his terms ... His record will surely never be eclipsed. Simply astonishing to read through his managerial record of achievements." - Former England, Liverpool and Manchester United striker Michael Owen.
"Alex has proven time and time again what a fantastic manager he is but he's also a wonderful person. His determination to succeed and dedication to the club have been truly remarkable," - Joel Glazer, joint chairman of the Manchester United board.
"He's done the lot, you will never see anyone of his kind again." - Former England and Manchester United midfielder Paul Ince.
"The game of football will be a lot poorer place without him. He has been quite simply the best. He followed in Sir Matt Busby's footsteps and even surpassed him. He will also be the toughest act to follow." - Professional Footballers' Association chief executive Gordon Taylor.
"I'm disappointed, shocked and very, very sad." - Former Manchester United goalkeeper Peter Schmeichel.
"Sir Alex Ferguson is one of the greatest British managers of all time and has been an incredible servant to Manchester United, bringing the club unprecedented success domestically and in Europe. His enthusiasm for our national game is boundless and I congratulate him on a remarkable managerial career." - British Sports Minister Hugh Robertson.