Arsenal have confirmed that a fan was arrested after throwing an object towards Manchester United's bench after the visitors' opening goal at the Emirates Stadium on Saturday.
United moved back into the top four following their 2-1 win in north London after an Antonio Valencia shot deflected in off Kieran Gibbs and a Wayne Rooney strike, before Olivier Giroud got one back late on.
Arsenal have promised stiff action against the fan if he is found guilty.
An Arsenal spokesman told Press Association Sport: "A male was arrested at the stadium for throwing an object towards the Manchester United bench after the first Manchester United goal.
"There will be full co-operation between Arsenal and the Met Police - we do not tolerate anti-social behaviour. If found guilty, individual will also be banned by club."