He used the moving hit to pay tribute to those who died, but became choked up during the performance, leaving it to the 50,000 fans at his show to sing for him.
Robbie was seen fighting back tears in videos and pictures posted on social media by fans at the Etihad Stadium on Friday.
Fan Stuart Rowson wrote alongside his video: “A tearful Robbie Williams struggles to sing Angels – after dedicating it to the victims and families of the Manchester attacks.”
Another clip was shared by David Raven, who said: “Emotional Robbie Williams dedicates Angels to Manchester terror attack victims, breaks down, 50,000 finish it for him. Everyone now crying.”
At the start of the concert, Robbie said: “We are Manchester and we’re not f****** scared.”
Robbie will join the line-up of stars at the One Love Manchester benefit concert this evening, raising funds for the victims of the terror attack at Ariana Grande’s Manchester Arena show.
The concert looks set to be one of the most-watched in history.
It will be broadcast in the UK on BBC One and BBC radio and shown live in more than 50 countries across five continents on BBC Worldwide.
Stars joining Ariana at the Emirates Old Trafford Cricket Ground include Justin Bieber, Katy Perry, Coldplay, Take That, Pharrell Williams, Miley Cyrus, Black Eyed Peas, Usher, Little Mix and One Direction’s Niall Horan.
Coverage, which starts at 6.55pm, will be presented by Sara Cox, Ore Oduba, Nick Grimshaw and Anita Rani.
● The We Love Manchester Emergency Fund had reached £7.2 million last night. To donate, visit redcross.org.uk/love or call 0300 456 4999.