Lisbon: Portuguese prosecutors said Tuesday they had opened an investigation after footage emerged of a police officer beating a man outside a football stadium as the man's children watched nearby.

The investigation was launched "following a complaint of alleged abuse of power during police intervention," the Porto attorney general said in a statement. Jose Magalhaes, a 43-year-old businessman, was beaten by a police officer as he was exiting the Afonso Henriques stadium Sunday in the northern city of Guimaraes, following Benfica's title-clinching Portuguese league victory.

The scene played out before Magalhaes's children, ages nine and 13. The younger one, in a state of shock, broke down in tears and was detained by two other policemen. The grandfather, who tried to intervene, was also accosted by police. A video of the incident was posted online, causing public outcry. Magalhaes was put under investigation for insulting public officials, a charge he has repeatedly denied.

"I just said to the agent that the police were not doing their work, because they would not let us leave the stadium. I was worried for the safety of my children," Magalhaes told journalists. The Portuguese police have begun disciplinary procedures against the officer, and the interior ministry launched an investigation into the incident.

The opposition party also demanded that the government account for the actions of the police. "The government needs to shed light on the behaviour of security forces," said Cecilia Honorio, a spokeswoman for the Left Bloc party in Portugal. The president of the Benfica team, Luis Filipe Vieira, denounced "the violence" of the incident and invited Magalhaes and his family to the league trophy ceremony on Saturday at the Luz stadium in Lisbon.

The festivities for Benfica, which won the national championship Sunday for the second year in a row, were tarnished by clashes between fans and security forces in Lisbon and northern Portugal. At the end of the decisive match between Benfica and Vitoria Guimaraes (0-0), Benfica supporters vandalised the stadium and ransacked the bar. The damages have been valued at more than 100,000 euros ($111,000).