Police and fans have been injured as Celtic and Rangers supporters clashed during a pitch invasion after an Old Firm match in Glasgow.
A number of arrests were made during what Police Scotland described as “despicable” and “shameful” scenes at Ibrox stadium.
Celtic fans initially spilled onto the pitch in celebration after a 4-2 win on penalties in the Scottish Cup quarter final before Rangers fans also joined the fracas.
Flares were thrown as police and stewards formed a barricade to separate the two groups of supporters.
Ch Supt Kate Stephen said the behaviour of a number of fans “must be condemned by everyone involved in football and wider society”.
She said: “A number of arrests have already been made, and Police Scotland will now work with both clubs and the Scottish Football Association to carry out a robust investigation following the scenes on the pitch at the conclusion of the match.
“Officers and stewards were faced with extreme hostility and violence over a sustained period, with many individuals having armed themselves with items clearly intended to cause harm.
“Officers and members of the public have been injured in this despicable display and I want to express my thanks to all officers and staff deployed.”
Reuters: Police and stewards formed a barricade between the two groups of fans who had invaded the pitch
PA Media: Fans wearing black clothing and blue and red face coverings ran across the pitch
The disorder quickly settled down with fans returning to the stands as police and stewards formed a barrier between opposing supporters.
It was the first time Celtic supporters have been given the full Broomloan Road Stand allocation at Ibrox since it was reduced to fewer than 1,000 in 2018.
Traditionally, around 7,500 Celtic fans filled the stand for Old Firm derbies.
Since then, away allocations at both Ibrox and Celtic Park have fluctuated between about 750 and zero in a series of tit-for-tat moves.
Police said some of the supporters had entered the Broomloan Stand without tickets, forcing delays to the security operation with turnstiles closed for a time.
PA Media: Green smoke could be seen over the Celtic end of the stadium
The Scottish Football Association condemned the supporters who went onto the pitch and said an investigation would be carried out “immediately”.
Scotland’s victims and community safety minister Siobhan Brown said the scenes were “unacceptable”.
She said: “Of particular concern is the reckless use of pyrotechnics which could have caused serious injury.”
Brown highlighted that the Scottish government had recently consulted on extending powers to ban football fans from games and was “considering next steps”.
Currently Football Banning Orders (FBOs) are only imposed if supporters are involved in violence or disorder – they enable the authorities to stop fans attending games anywhere in the UK for 10 years.
Both clubs told BBC Sport they would not comment officially yet.
In a post-match interview, Celtic manager Martin O’Neill said: “You’re asking me a lot of political questions in this moment when I’m in a euphoric state.
“The fans have spilled over, big scenes at the end, all over the place. I’m not diminishing it at all, I will have a think about it and speak to you in due course.”
Meanwhile Rangers head coach Danny Rohl told Premier Sports: “I’m not sure what was at the end, I was in the tunnel.”