Rangers fans will want to forget Saturday's embarrassing 5-1 defeat against Celtic as quickly as possible - for more reasons than one.A combination of woeful defending and lethal finishing saw Celtic extend their unbeaten run at Ibrox and humiliate their Old Firm rivals.Myles Beerman's reckless challenge on Patrick Roberts to give away a penalty in the sixth minute set the tone for the afternoon.Scott Sinclair duly dispatched the spot-kick to give Brendan Rodgers' side an early lead.A screamer from Leigh Griffiths made it two-nil 12 minutes later, the 26-year-old emphatically firing past Wes Foderingham - who should have done better - from a tight angle.It was all one-way traffic from then on, with Celtic adding third and fourth goals through Callum McGregor and Dedryck Boyata with just over an hour played.A customary goal for Kenny Miller lightened the mood at Ibrox but no sooner were Rangers fans given reason to smile, they went 5-1 down.Mikael Lustig tricked his way past Clint Hill in the 87th minute before finding the bottom-right corner with an inch-perfect finish.A day to forget for the Gers, then, but it could be about to get a lot worse.Shocking footage has emerged of a Rangers fan appearing to racially abuse Sinclair with a monkey gesture as he celebrated his opener. Check it out below.

Disgusting behaviour. Football fans were appalled by what they saw and tweeted Police Scotland's official Twitter account asking them to investigate the incident.

They responded: "Our social media feeds cannot be used to report crimes or incidents. If you wish to report a crime please call 101 to report."

TWITTER REACTS

The fan in question has yet to be identified but judging by how clear the footage is, it's likely he will be caught and punished by Rangers.

Racism has no place in football - or anywhere for that matter - yet it still happens all too often. A very sad reality.