A controversial goal from Lazio's Felipe Anderson has led to Inter Milan's first defeat of the Serie A season while AC Milan secured a comeback win over Verona to go top of the table.
Anderson scored in Lazio's 3-1 win while Inter fullback Federico Dimarco was down injured.
The goal sparked an angry reaction from the Inter players who were calling for Lazio to kick the ball out of play - instead Anderson redirected a rebound of a shot from Ciro Immobile into the net after a counter-attack from the hosts.
Sergej Milinkovic-Savic then secured the three points with a header in stoppage time as Inter's focus was lost.
Immobile had cancelled out Ivan Perisic's 12th-minute penalty with a penalty of his own in the 64th.
Scuffles between players reignited after the full-time whistle with Lazio defender Luiz Felipe shown a red card for his involvement.
The game marked the return to the Stadio Olimpico for Inter Milan coach Simone Inzaghi, who had coached Lazio for the past five seasons after also spending the bulk of his playing career with the Roman club.
"We lost our heads," Inzaghi acknowledged after the game.
"That shouldn't happen, because the referee can let play continue and there were 10 minutes plus stoppages to go."
Lazio moved up to fifth while Inter remained in third place on 17 points, five points behind AC Milan - who defeat Verona 3-2 in a pulsating encounter - with Napoli - who host Torino on Sunday - second after seven wins from as many games.
At the San Siro, an own goal from Koray Gunter completed Milan's second-half fightback.
Gianluca Caprari and Antonin Barak put the visitors ahead 2-0 after 24 minutes but they could not hold their lead.
Olivier Giroud headed home in the 59 minute before Franck Kessie converted a 76th-minute penalty and Gunter redirected a cross into his own goal in the 79th.
"I saw the right expressions on the faces of the players at halftime, we had to step it up and we did. It was a great test of maturity," Milan coach Stefano Pioli told Sky Italia.
The winner came just after 40-year-old Zlatan Ibrahimovic came on to mark his return from injury.
"Everyone knows his presence is very important to us. He is not yet in top condition, I hope he can improve and play a lot of games," Pioli said.
Earlier on Saturday, Spezia came from a goal down to beat Salernitana 2-1 and climb out of the drop zone.
It's the first Serie A weekend in the pandemic when stadium capacities have been raised to 75 per cent.
Australian Associated Press