A controlled explosion has been carried out at Old Trafford by bomb disposal experts, Greater Manchester Police said.
The stadium had been evacuated and Manchester United's last-day Premier League match cancelled after a suspicious package was discovered at the grounds.
A suspicious item which sparked the evacuation "was a training device which had accidentally been left by a private company following a training exercise involving explosive search dogs", Greater Manchester Police said.
Bomb disposal experts carried out the explosion on what was described as "incredibly lifelike explosive device".
Greater Manchester Police said after the evacuation: "Full assessment now concluded and found device wasn't viable. A full search of the stadium is ongoing."
Fans on the Stretford End and the Sir Alex Ferguson stand were ordered to leave and players, who were already on the pitch warming up ahead of the 3pm kick off, were sent back down the tunnel after the find.
At around 2.40pm, an announcement was made for security personnel to invoke "operation red code" and it soon became evident fans were leaving the South Stand and Stretford End.
The match against Bournemouth has been abandoned. MUFC said the package was found in the north west quadrant of Old Trafford.
Police walked sniffer dogs through the ground and the army's Bomb Disposal Unit is at the scene.
It is the first time a match has been abandoned at the ground since 2004, the Manchester Evening News reported.
The club said: "Due to the discovery of a suspect package in the north-west quadrant of the ground, the match has been abandoned for today on police advice.
"People in the stadium are advised to remain in their seats while the forecourt is cleared of fans evacuated from the stadium. Further announcements will be made as soon as possible."
Fans gathered outside the ground having been ushered out by security and police.
One fan had travelled all the way from Sierra Leone to see Manchester United play at their home ground for the first time, according to the vice-chair of the club's supporters trust.
Ian Stirling tweeted:
In a statement, the Premier League said the game would be rearranged for "as soon as practically possible".
The statement said: "The decision to abandon the match was taken after police advised of the necessity to deal with a suspect package.
"When it comes to security, it is obviously right that Manchester United and the Premier League place the safety of supporters and employees foremost.
"The Premier League will seek to rearrange the fixture as soon as is practically possible and will advise fans accordingly. It is always the last resort to abandon one of our fixtures and while we apologise for the inconvenience caused to fans we are sure, in the circumstances, they will appreciate the need to do so."