RIOT police used CS spray to control rival fans as trouble flared at Wednesday night's clash between Hearts and Celtic.
Police said it was used in two separate incidents before and after the bad-tempered game at Tynecastle.
Mounted police had to keep opposing fans apart as skirmishes broke out around the ground in the build-up to kick-off.
The hooliganism came just days after Hibs and Hearts fans marred the Edinburgh derby by throwing bottles, coins and flares on to the pitch at Easter Road.
Last night, a police source said: "The CS spray is only used as a last resort, but the officers clearly felt the violence would escalate if the situation was not brought under control quickly.
"A skoosh of that stuff would stop an elephant in its tracks and probably helped prevent a full blown riot in both incidents."
Trouble started before the 7.30pm kick- off as supporters clashed in Russell Road, where away fans often park their buses. It is understood a cop used the spray on at least one supporter.
And a second cop was again forced to use the spray during running battles in McLeod Street as fans poured out of the ground following the match, which Hearts won 2-0. Police are also investigating reports that a Celtic fan was slashed in a bar.
And dozens of police were involved in a stand-off with up to 20 Celtic fans in the concourse of the Roseburn stand.
It's believed the confrontation started after cops kicked out a Celtic supporter from the visitors' part of the ground.
Police say they had to eject five fans in total during the game.
An inquiry will also look into reports that seats were smashed by visiting supporters, as well as vandalised catering stalls and toilets.
A police spokesman said there were two arrests at the match but more could follow.
He could not confirm the extent of any damage inside the stadium and said the force would liaise with Hearts.
The spokesman added: "Police were required to deal with a number of incidents of disorder before, during and after the game.
"Inquiries are under way in order to identify those responsible for the disorder, and anyone who has any information that can assist should contact us."
The police source said: "There can often be trouble when these two teams meet, but there was a lot of bother even for a Hearts and Celtic game.
"There were times when things looked like they could easily get out of hand before and after kick-off and there were a number of incidents inside the ground as well."