Departing Arsenal boss Arsene Wenger has been described as "one of the greatest Premier League managers" by old rival Sir Alex Ferguson.
The 68-year-old Frenchman will leave at the end of the season after nearly 22 years at the club, 12 months before his latest contract was due to expire.
Former Manchester United boss Ferguson paid tribute to his "professionalism, talent and determination".
Ex-Gunners striker Thierry Henry said Wenger leaves "an untouchable legacy".
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Wenger won three Premier League titles and a record seven FA Cups, including the Double in 1998 and 2002.
"I am pleased he has announced he is leaving at this stage of the season as he can now have the send-off that he truly deserves," said Ferguson, who won 13 league titles in 26 years at Old Trafford, before retiring in 2013.
"He is, without doubt, one of the greatest Premier League managers and I am proud to have been a rival, a colleague and a friend to such a great man.
"It is great testament to his talent, professionalism and determination that has been able to dedicate 22 years of his life to a job that he loves."
Gunners goalkeeping legend Bob Wilson said Wenger was Arsenal's greatest boss, and broke down in tears as he told BBC Radio 5 live how the Frenchman had helped his family.
Wilson, whose 31-year-old daughter Anna died of a rare form of cancer in 1998, said: "I was there on the day he arrived and he is without doubt one of the three greatest men I have ever met in my life.
"It is not just his football knowledge and the way he changed the game in this country but as a human being through difficult times for me and my wife.
"Arsene is not only the greatest manager in Arsenal's history, he has personally changed the face of the game in this country.
"We had a team who enjoyed a drink and he changed everything about psychology, rehydration, training and everyone followed suit."
Club captain Per Mertsacker said he felt "sad and emotional" on hearing of Wenger's departure.
"There have been a lot of challenges, a lot of highs and lows, but how he went through tough times has taught me a lot about his character and about how to trust people and get the people in the right places to be successful," he said.
The Gunners are sixth in the league and set to miss out on a top-four spot for the second straight season, with their hopes of reaching the Champions League resting on winning the Europa League.
Henry said it would be "an amazing achievement" if Wenger could sign off by winning a trophy.
"His legacy is untouchable. Managers, fans from other teams - (they talk about) how Arsene changed Arsenal," he told Sky Sports.
Wenger had been under increasing pressure to step down from some fans having failed to win the league for 14 years.
Former Arsenal midfielder Stewart Robson said he was like a "dictator" and should have left several years ago.
"You can't take away the good things when he came he changed the philosophy of Arsenal," Robson told BBC Radio 5 live.
"He was fantastic in that period but, like a lot of dictators, they think they are going down the right road - but even when people are saying you need to change, he wouldn't change.
"Arsenal under Arsene Wenger in the last few years have been too one-dimensional and behind the times in recruitment and physiotherapy. It is the best thing that could happen to Arsenal that he is leaving now."
Appointed on 1 October 1996, Wenger is the Premier League's current longest-serving manager and has taken charge of a record 823 games.
His team were dubbed "The Invincibles" in the 2003-04 season after going unbeaten in their title-winning campaign.
Reaction - 'He made us change the way we played'
Arsenal midfielder Jack Wilshere said: "Always a gentleman, like a father through tough times in my career. He always believed in me when most people didn't. Thank you for everything boss. It's down to us now."
Ex-Gunners goalkeeper David Seaman told BBC Radio 5 live of the impact Wenger made when he arrived at Arsenal.
"He changed the way we played, the way we ate, the massage routines, stretches. He was wise enough not to touch the back four and went on to success and more success with the Double and Invincibles," said Seaman.
Former Arsenal striker Robin van Persie said: "Thanks Arsene for all your confidence in me over the years. You are a very special man and a top coach. Definitely one of the very best ever."
Spain and Chelsea's Cesc Fabregas, who burst in to the Arsenal starting line-up under Wenger, posted on Instagram: "Wow. I never expected that but it shows the great dignity and class of the man. I will never forget his guidance and support, his tutelage and mentorship."
Former Arsenal striker John Hartson, who played in Wenger's first match, told BBC Wales Sport: "English football and the Premier League owe him an awful lot.
"The stretching before matches and training, getting rid of the lactic acid, jogging around for 20 minutes before you even started - that had never been done before.
"Arsene brought all these new ideas and then an influx of foreign managers arrived in the Premier League."
Former Arsenal captain Tony Adams posted on Instagram: "Thanks for everything Arsene. Move over Herbert (Chapman), Arsene Wenger the greatest Arsenal manager."
Match of the Day presenter Gary Lineker said Wenger had given much of his life to Arsenal. "He has brought much success to the club. He has given much to the game of football in our country. Merci et bonne chance, Monsieur Wenger," said the former England striker.
Ex-Manchester United defender Gary Neville said: "Arsene Wenger built the best teams that I played against in English football.
"The '98 team was amazing. The biggest compliment is that he played football that made us change the way we played against them. He now deserves the most incredible send-off from all in the coming weeks."
Chelsea manager Antonio Conte said: "He deserves great respect not only from the Arsenal supporters but the whole world of football. We are talking about one of the best managers in the world. It is very difficult to see another manager stay such a long time in the same club. "
Wenger had earned huge respect throughout the game, said Premier League chief executive Richard Scudamore.
"All of Arsene's teams have been a joy to watch and his 2003-04 'Invincibles' will go down as one of the best in English football history," he added.
Arsenal Fan TV presenter Robbie Lyle said: "Everyone knows this season the fans had really turned against Arsene Wenger, even fans who in the past have been really behind Wenger have said it is time for him to go.
"Even so it has been a real surprise. He did a press conference yesterday and there was no hint. It is the end of an era. There are lots of fans who have grown up and all they have known is Arsene Wenger. It is a new chapter and is exciting in a way. I also think it is a major moment for the board and owners of Arsenal. We remember what happened with Manchester United. Arsenal have to get this right."
Former Borussia Dortmund manager Thomas Tuchel has already been linked with the job, while Wenger himself said former Arsenal midfielder Patrick Vieira "has the potential" to succeed him.
Tuchel was the early favourite with bookmakers, ahead of Germany boss Joachim Low and former Real Madrid, AC Milan and Chelsea manager Carlo Ancelotti.
"It's a special job, a special club. The infrastructure, the training ground is out of this world, as good as anything in Europe," said Hartson.
"In terms of the next manager to step in, Arsenal don't change their managers around willy-nilly. They don't get two or three years. Arsene Wenger had 22 years, before him there was George Graham - a long, long time in the job.
"Arsenal will take their time. They have until the end of the season and the summer. They will want to get someone in of course to take care of pre-season and their tour before the season kicks off. It's a brilliant club - for me it's the biggest club in London."