Richard Lewis, 44, claims to have been attacked in Lisbon on Saturday 29 May.
Mr Lewis, an IT consultant who previously worked in Portugal for six years, had returned to the country for a holiday, and was drinking in Lisbon's busy Bairro Alto area with a friend, Robert Drayson, when the alleged incident occurred.
"We were standing outside a bar with our drinks when a policeman suddenly arrived and handcuffed a woman," said Mr Lewis.
"Within seconds, a group of other police officers arrived and began moving people away from the scene. I don't know what the woman had done, but they were using what seemed to be unnecessary force against her, pushing her to the ground. Onlookers were visibly shocked, and Robert took a few photos on his phone."
Mr Drayson was immediately asked to hand over his phone, and when he refused, Mr Lewis said the police became increasingly aggressive.
"I speak Portuguese, so I spoke to the police very calmly, explaining that my friend needed his phone for his work, and that he wasn't involved in what was going on. They totally ignored me. Eventually, however, the officer holding my friend loosened his grip, and I turned to go.
"It was at this point that the officer swung his baton directly in my face."
Mr Lewis was knocked onto the floor by the force. "The blow had split my nose, breaking it, and the bones were protruding through. None of the police checked to see if I was dead or alive, and no one called an ambulance."
Mr Lewis's story was afterwards widely reported in the Portuguese media.
"I lived in Portugal for many years, and I have more friends there than I do in Britain. But in all my time there, I never saw the police act like this," he said.
"Expats and tourists are always told to be on the lookout for pickpockets and the like, but I would warn them now to be careful around the police too. This happened on a busy night, in an area roughly equivalent to London's Soho, and to an innocent person.
"The men who came into that street were just thugs in uniforms."
A spokesman from the Foreign and Commonwealth Office said that they had little information on the case, but "we have the media reports and stand ready to provide consular assistance if requested".
A spokesman for the Portuguese police said that the subject was under criminal investigation and that according to Portuguese law, no further information could be given.
They added however: "This matter is also being analysed by our internal affairs in order to determine if there was any kind of misuse of police force and apply the correspondent disciplinary measure."