Sir Alex Ferguson received a guard of honour before his last home match after 26 years in charge of Manchester United.
The Impossible Dream rung around Old Trafford as fans waved red flags in honour of a manager who has won 13 league titles for the club.
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Two knives were found at the scene in Islington, North London, and the suspects fled on foot before being arrested.
The violence began after a counter-protester allegedly doused a female rally organiser with pepper spray.
As temperatures climb through the day, the warmest spots are likely to be across western parts of Northern Ireland and western Scotland.