Cesc Fabregas was shown a yellow card after tumbling in a challenge with Southampton defender Matt Targett, infuriating Morinho whose players have been accused of diving in several recent incidents.
"I think it is a scandal because it is not a small penalty - it is a penalty like Big Ben," Mourinho told the BBC.
"In this country - and I am happy with that, more than happy with that - we will just say that it was a big mistake with a big influence in the result.
"I am happy that it is this way, with respect for the referee. He made a big mistake like I make, like the players make sometimes."
Chelsea's Branislav Ivanovic, Diego Costa and Gary Cahill have all been involved in controversy over diving in recent games and Mourinho believes there is a conspiracy against his team.
"They are not incidents," he said. "That's a campaign, that's a clear campaign. People, pundits, commentators, coaches from other teams - they react with Chelsea in a way they don't react to other teams.
"They put lots of pressure on the referee and the referee makes a mistake like this. We lose two points, Fabregas earns a yellow card."
Southampton manager Ronald Koeman was not sure about the decision.
"If the referee thinks it's simulation then you have to give the player a yellow card," said the Dutchman whose team have consolidated fourth place in the table following a mini-slump in form.
Southampton took a deserved lead through Sadio Mane but Chelsea equalised just before halftime with a well-taken goal by Eden Hazard and remained three points clear at the top following Manchester City's shock 2-2 draw at home to Burnley.
"Of course, one point against Chelsea is maybe three points against another opponent," Koeman said.
"I am proud of the players, the organisation in the team, the spirit in the team. We knew it would be very difficult but the boys did it fantastic.
"I was a little bit disappointed because we conceded before halftime, but we stayed organised in the team.
"We got a little bit of lucky but you need that against these good teams to get a result."
(Reporting by Ed Osmond, editing by Pritha Sarkar)
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