Two of the Old Trafford stands were evacuated about 20 minutes before the scheduled kickoff of the Premier League game against Bournemouth at 1400 GMT when the item was found in the toilets at the ground. The match was called off soon afterwards and the whole 75,000-seater stadium cleared.
TV footage showed fire engines arriving at the stadium, one of the world's most famous football grounds.
Police used sniffer dogs to search the area before calling in an army bomb disposal unit which carried out a controlled explosion on the item they described as being "an incredibly realistic-looking explosive device".
"Following today’s controlled explosion, we have since found out that the item was a training device which had accidentally been left by a private company following a training exercise involving explosive search dogs," Assistant Chief Constable John O’Hare from Greater Manchester Police said in a statement.
"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."
Manchester United said in a statement on their website that the package had been found in the North West Quadrant, adding that the match was abandoned on police advice.
United Executive Vice Chairman Ed Woodward said staff were regularly trained in security with the police and emergency services. The club will investigate the incident fully.
The game has been rearranged for Tuesday. United cannot now qualify for next season's Champions League after Manchester City drew at Swansea City on Sunday.
United are sixth in the Premier League table and would move above Southampton into fifth with a draw or victory against Bournemouth. They are due to play Crystal Palace in the FA Cup final on Saturday.
On Wednesday Britain raised the threat level from dissident Northern Ireland militants to "substantial", meaning an attack on the British mainland is considered a strong possibility.
There have also been concerns that the Belgian-based cell of Islamic State that carried out attacks in Paris six months ago and in Brussels in March might have been involved in plots in Britain.
(Reporting by Ed Osmond and Kylie MacLellan; Writing by Kylie MacLellan; Editing by Louise Ireland)
