Cristiano Ronaldo smashed a car window as he lashed out in fury at a teenage girl videoing him in the street.
The24-year-old former Manchester United star kicked the window, sending shards of glass into 17-year-old student Sara Pardal, it was claimed by Portuguese media.
Sara was treated in hospital for her alleged injuries and later reported the attack to the police.
The teenager reportedly told police she was sat in the car outside Ronaldo's Lisbon apartment because she is a fan and wanted an autograph.
The FIFA World Player of the Year has admitted kicking the car in a rage - but insisted the girl was working with a paparazzi photographer.
He has accused her of exaggerating her injuries.
Ronaldo, who has just completed his £80 million move from Manchester United to Real Madrid, said: 'I regret what I did, but I can't promise that I would not react in the same way again in the same circumstances.'
The incident took place on at around 9pm on Sunday in Lisbon, where the footballer is supposed to be relaxing before being presented as a Real Madrid player in Spain next Monday.
Ronaldo was heading to an Elton John concert in the Portuguese capital with his mum Dolores Aveiro, 54, sisters Elma and Catia, and a group of friends.
The star claims the girl was with a photographer who had been stalking him, 'standing for days on end in front of my house, eating yogurt and waiting for anything newsworthy to happen.'
He says the photographer and the girl had followed him to his house from the Ritz Hotel.
When he spotted the girl filming him, he leapt out of his £200,000 Ferrari 599 GTB Fiorano in a rage and kicked the car.
Ronaldo said: 'This persecution caused my mother to be so disturbed and upset that I had to stop my car to try to convince them to leave us in peace.'
His management company admitted in a statement: 'Faced with Dolores' distress, Cristiano Ronaldo ended up reacting impulsively, kicking the car containing the individual in question and his travelling companion.'
The footballer added: 'When I'm on my own, fine, I try to swallow it and remain indifferent. But when my mother is involved, then I'm sorry but I don't let anyone, I repeat anyone, harass her.
'When my mother is involved it is very difficult for me to keep a calm sate of mind.'
Portuguese newspaper Correio da Manha described the girl as a fan who had been trying to get an autograph of Elton John at the Ritz, then went to Ronaldo's house hoping to meet the footballer.
But rival daily 24 Horas said she was in the car with freelance photographer Hugo Martins, 35.
Ronaldo has been followed closely by photographers in his native country since returning from holiday in the U.S.
Sara went to the Sao Jose Hospital for treatment to cuts on her arm, arriving at around 11pm on Sunday night.
A spokeswoman for the hospital said: 'We are not allowed to disclose any information about our patients without the consent of the patient.
'We cannot confirm even that she was treated here in the hospital.'
The following day at around 5.50pm she is said to have reported the incident to the Public Security Police.
A police spokesman said: 'We cannot make any comment whatsoever. We will not be confirming or denying these stories.
'You will have to ask the girl or Cristiano Ronaldo what happened.'
If state prosecutors decide Ronaldo has a case to answer, he could face charges of wounding or destroying property.
He could be ordered to pay compensation to his victim, who could also pursue a civil claim.
The girl's father Manuel Pardal, 78, said from the family home in Lisbon: "I've been told I have to cut off the phone. Goodbye.'
Ronaldo claims the photographer later offered to sell him a video of the incident.
The footballer said: 'He immediately tried to sell the video recording to the company that represents me, but I could never, I emphasis, never, agreed.
'During the attempt to negotiate the sale of the video, the girl companion, although it could clearly be seen she had no visible injury, appeared wearing a hospital gown, which leads me to believe they planto add various items to the small encounter that actually took place.
'Unfortunately, I know that one day soon everything they recorded will be available in every newspaper and what they falsely add will be credited, because unfortunately there are always buyers, there are unscrupulous people and there will continue to be rewards for this reckless attempt to try everything, at any cost, for personal gain.
'That's not my way. I could have bought the tape and the subject would never have been made public, but I know it will be made public.
'Because that's what I want, because this is the only way I know how to be in life.'
Asked if Ronaldo admitted smashing the car window in the incident, the footballer's management company said: 'We are not commenting other than the statement already put out.'