Former Scotland striker Law's death was confirmed on Friday. He was 84 and had been diagnosed with dementia in 2021.
Flowers, scarves and other tributes were laid outside United's stadium in front of a statue commemorating Law, Bobby Charlton and George Best - the club's so-called 'holy trinity' - on Sunday before the Premier League game against Brighton.
Denis Law was the Scot who United fans all called 'The King'. (AP PHOTO)
Before kick-off, a piper led out the teams playing "Flower of Scotland."
Former United manager Alex Ferguson laid a wreath in the centre of the field and was joined by Law's former teammates Paddy Crerand, Alex Stepney and Brian Kidd.
Law scored 237 goals in 404 appearances for United and was the only Scottish player to win the Ballon d'Or.
"He was the best Scottish player of all-time," Ferguson said. "He summarised a Scotsman, the fighting element, he could fight in an empty house! An incredible human being."
A poem about Law's career was read out and followed by applause from the crowd.
Tributes to Law were laid next to his two statues at Old Trafford. (AP PHOTO)
Ferguson added: "He was a fantastic player and an incredible human being. The great thing is that he did not use his fame, he was so level-headed and down to earth and we are talking about a guy who had all of these caps for Scotland and is called the 'King of Old Trafford'.
"It is an interesting thing because why did they call him the King? You had guys like George Best, Bobby Charlton - the greatest English player of all-time and a fantastic player for our club - and they called Denis 'the King'.
"It was something to do with the determination and the will to win every time he went on that football field. He was a really competitive human being and it was a natural thing for the supporters to associate with him as their King."
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