A security company said to have accidentally left a dummy bomb at Old Trafford that triggered an evacuation is facing calls for a full inquiry.
Greater Manchester's mayor and police and crime commissioner, Tony Lloyd, branded the scare a "fiasco" that had put people in danger.
Bomb disposal experts were called to the 75,000-seat stadium on Sunday afternoon to carry out a controlled explosion on the "incredibly lifelike" training device that sparked a red alert.
The fake bomb was found in a toilet just minutes before Manchester United were due on the pitch, leading to the Barclays Premier League game against Bournemouth to be postponed.
Police said it had been left by a "private company" after an earlier drill involving search dogs.
Fans praised counter-terror police and security staff for their response that saw two stands evacuated and the players kept in their dressing rooms by security.
However, Mr Lloyd said it was "outrageous" the situation had arisen and called for an urgent inquiry into "how this happened, why it happened and who will be held accountable."
He said: "This fiasco caused massive inconvenience to supporters who had come from far and wide to watch the match, wasted the time of huge numbers of police officers and the army's bomb squad, and unnecessarily put people in danger, as evacuating tens of thousands of people from a football stadium is not without risk.
"Whilst this in no way demeans the professionalism of the police and stewards responsible for getting the fans out, or the supporters' calmness and cooperation during the evacuation, it is unacceptable that it happened in the first place."
Ed Woodward, United's vice-chairman, said the club will "investigate the incident to inform future actions and decisions".
The evacuation of the Stretford End and Sir Alex Ferguson Stand began around 20 minutes before the scheduled kick-off of 3pm after an announcement was made for security personnel to invoke "operation red code".
Shortly after 3.15pm those remaining in the stadium were informed the game was off.
Army bomb disposal experts called in by officers carried out a controlled explosion on the device in the North West Quadrant of the ground at 4.30pm.
Assistant Chief Constable John O'Hare from Greater Manchester Police (GMP) confirmed the item was a "training device which had accidentally been left by a private company following a training exercise involving explosive search dogs".
"Whilst this item did not turn out to be a viable explosive, on appearance this device was as real as could be, and the decision to evacuate the stadium was the right thing to do, until we could be sure that people were not at risk."
The game was called off after discussions between the Football Association, the Premier League and police and has been rearranged for Tuesday at 8pm, four days before United play Crystal Palace at Wembley in the FA Cup final.
It is understood that, as a goodwill gesture, United will offer supporters a refund as well as a free ticket for the rescheduled game.
They were hoping a win against AFC Bournemouth would lead to Champions League qualification - an ambition that now looks unlikely after rivals Manchester City drew with Swansea in their final fixture of the season.
The alert at Old Trafford came just days after GMP held a mock terrorist operation at Manchester's Trafford Centre which saw a fake suicide bomber shouting "Allahu Akbar" immediately before detonating a bomb.
The overnight operation had been testing the anti-terror police's response to a Paris or Brussels-type attack on civilians but was criticised after footage showed one actor repeatedly shout the religious phrase meaning "God is great".
GMP Assistant Chief Constable Garry Shewan issued an apology, calling it ''unacceptable'' to use the phrase.
In January, United reportedly beefed up security at Old Trafford in the wake of the terror attack at the Stade de France in Paris.
On Wednesday, Home Secretary Theresa May announced that MI5 had raised the threat level to Great Britain from Northern Ireland-related terrorism from moderate to substantial - the third most serious category out of five.
Mrs May said the move ''reflects the continuing threat from dissident republican activity''.
The threat level to the UK from international terrorism remains at severe - meaning an attack is ''highly likely''. This has not been changed.