A police officer is under investigation after video of him punching a woman in the face as he arrested her was posted online.

Footage shows the officer, who has not been named, hitting 20-year-old Dominique Difiore while clutching a stun gun outside a house in Chester, Pennsylvania, as St Patrick's Day celebrations got out of control.

Difiore tumbles to the ground where witnesses said she was shot with the stun gun, leaving her bleeding profusely from her ear.

Police in Pennsylvania have launched an investigation after one of their officers was filmed punching 20-year-old Dominique Difiore in the face during a St Patrick's Day brawl

Chester Police say Difiore struck a police sergeant before getting hit, and video does appear to show the woman raising her arm before being knocked down.

Nevertheless, the police department has opened an investigation into the brawl after footage was posted on Twitter, where it went viral.

Latine Bethea, who saw the brawl, told CBS Philadelphia: 'I was shocked. I was really shocked. I couldn't believe a man hit a female like that.'

Cameron Thomas, who also saw the fight, told what happened next: 'He dove to the ground, then tased her. That’s what made it excessive.'

Police say the incident began around 5pm Saturday when a group of people tried to get into a house party during St Patrick's Day.

Police say they arrested four people near Widener University after a fight broke out when a group of people were denied entry to a house party

The group was turned away, officers say 19-year-old Jaylene Westfall attacked two of the people who lived at the house.

The residents managed to lock Westfall out of the building but she kicked the door in before going inside along with other women and continued the attack, police say.

Westfall was arrested alongside Difiore and two others - Shawn Connelly, 21, and 20-year-old Tess Herman.

The brawl happened close to Widener University and reportedly after a Widener pub crawl, though the event was not sanctioned by the college.

None of those arrested were students of the university.

A statement from Chester Police said: 'A partial video of the event is circulating on various social media platforms.

'The incident, the video and the level of force used in connection with the event are all currently under investigation.'