Fans mob Tractor Sazi head coach Toni Oliveira on May 15, 2015, moments before celebrations turned sour when they realised they had not won the Iranian Premier League title after all (AFP Photo/Omid Vahabzadeh)

Tehran (AFP) - When the final whistle blew thousands of jubilant fans ran on to the pitch to celebrate what they thought was their team's first ever Iranian Premier League title.

But while the smiles were widening and with Portuguese coach Toni Oliveira being hugged by supporters, the celebrations suddenly turned sour as it emerged they were mistaken.

Oliveira's unlucky side, Tractor Sazi, from Tabriz in northwest Iran, had thought a 3-3 draw with Naft (Oil) of Tehran on Friday night was enough to clinch the league.

With just minutes to go Tractor Sazi fans believed their rivals Sepahan FC, from the central city of Isfahan, had only managed a draw when they needed a win to take the championship.

Having run down the clock in the game's dying moments, Tractor Sazi's quest for the league -- after two runners-up spots in recent years -- finally seemed over when the whistle blew.

But as pandemonium broke out on the pitch the game's TV anchor -- knowing Sepahan had in fact won their match 2-0 to take the title -- was left flummoxed and unable to explain the celebrations.

When reality dawned for Tractor Sazi's fans -- 90,000 were in a stadium whose official capacity is only 70,000 -- the mood changed, seats were smashed and fights broke out.

"We were tricked," Oliveira, a former Benfica boss who had run around the pitch in celebration only then to learn the bitter truth, told the official IRNA news agency.

Having seen his side slip from a 3-1 lead and lose two second-half goals when one of his players was sent off, Oliveira also blamed the referee for the ill fate that befell his side.

Pictures showed the 68-year-old coach, a former assistant at Benfica to ex-England boss Sven-Goran Eriksson before later taking the top job, crestfallen.

Social media in Iran was filled with people expressing sorrow for Oliveira, insisting his team must have been the victims of dirty tricks.

"Something bizarre happened," said Naft chief executive Mansour Ghanbarzadeh, trying to explain how joy had turned to outrage.

"We were following the other game on television in the changing room but at 87 minutes in, suddenly the TV, radio and cellphone networks blacked out and we had no means of communication."

"At that very moment, we were told the match between Sepahan and Saipa had ended with 2-2 draw," he added.

In what IRNA described Saturday as "a dark chapter that has damaged Iran's footballing reputation," however, Tractor Sazi's fans and players were left baffled and then angry.

"Even Alireza Mansourian -- Naft's coach-- congratulated Toni on his victory," said Ghanbarzadeh, calling for an investigation.

The brawls that broke out left some fans requiring treatment. The commotion also led to the players being locked in their dressing rooms for safety reasons. A refereeing official was also assaulted.