Lisbon - Benfica's celebrations at winning their 34th Portuguese title on Sunday were marred by post match clashes between police and fans, resulting in several injuries, police said on Monday.
Benfica won the league title after a 0-0 draw away to Vitoria Guimaraes combined with a 1-1 draw for nearest challengers Porto at Belenenses.
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It is the first time that Benfica have won back-to-back titles in 31 years and the third time they have won it under coach Jorge Jesus.
Fans took to the streets of Lisbon to celebrate, including the Marques de Pombal square, where Benfica's successes are usually celebrated.
Violence erupted at 1am in the square when rival groups threw bottles, stones and other objects at police, who tried to disperse the crowd with batons after the Benfica faction had begun singing their club song.
"It was chaos, I felt like I was in a Western. I'd never seen anything like it, the supporters were overexcited," said one Lisbon fan Rui Trindade.
Similar incidents also took place at the Afonso Henriques Stadium at Guimaraes.
Television images of one Benfica fan Jose Magalhaes, 43, being punched by a police officer in front of his children set the social media network alight.
The Interior Ministry announced they had launched a probe into the incidents, which were slammed by Benfica president Luis Filipe Vieira.
"We have to find out if there was not, at certain times, excessive force used by police," Vieira said in a statement.
Vieira also issued a strong message to the "minority" of fans involved in the incidents, demanding that "they be identified and punished", adding that "football does not need them".