Police made three arrests after pyrotechnics were set off during Crystal Palace's 2-1 win at Chelsea on Saturday.
Four loud bangs were heard coming from the area of the Shed End which housed visiting Palace fans, with smoke bombs and at least one flare visible.
Two people were arrested in the visiting end for allegedly setting off the first pyrotechnic, which followed Wilfried Zaha's equaliser in the London derby, the Metropolitan Police said.
A third person was arrested for allegedly assaulting a steward who was attempting to deal with the incident.
There were five arrests in total around the match at Stamford Bridge, with two relating to possession of Class A drugs, the Met said.
Two loud bangs followed Zaha's ninth-minute equaliser after Cesc Fabregas had put Chelsea in front.
And a third followed when Christian Benteke struck in the 11th minute with a goal which ultimately proved to be the winner.
There was a fourth, with a red flare visible soon afterwards, just prior to the final whistle being blown.
Chelsea and Palace could face FA scrutiny over the incidents.
Police said much of their work prior to the match was related to pyrotechnics, while bag searches and sniffer dogs are present on match days at Stamford Bridge.