Police Commissioner John Timoney was injured when hundreds of demonstrators disrupted traffic in the centre of Philadelphia on Tuesday and clashed with police, resulting in 285 arrests.
Mr Timoney, who was born in Dublin and has been an adviser to the Garda Siochana, was treated in hospital after he and two fellow officers took on a group of 30 protesters who were overturning a car.
"We came around and jumped them," the police chief said later. "Big fight. I had a big guy wrapped around me. All of a sudden the rest of them kind of turned on us, took my bike, whacked me."
Mr Timoney, who has been in the streets each day on his bicycle overseeing his officers, said he was "punched, kicked and hit with my bike . . . I got banged up. I'm OK. I'll live."
But police officer Ray Felder, who was with Mr Timoney, was detained in hospital with serious head injuries.
The city police have been widely praised for their restraint in dealing with the demonstrators and for avoiding the kind of confrontations witnessed in Seattle last December when a curfew had to be imposed. Mr Timoney is credited with ensuring the protests did not become riots.
The street protests flared up on late Tuesday afternoon after two days of peaceful demonstrations. Most of the action took place in the city centre, well away from the sports stadium where the Republican Convention is taking place.
The demonstrators, representing a variety of causes including the anti-death penalty movement, kept the police on the move as they switched their sitdowns from street to street. They lay down at junctions, joined to each other by a device of chains and vinyl pipe called a "Sleeping Dragon" which made it very difficult for police to arrest them.
Other groups with anarchist insignia rampaged, attacking police cars and overturning heavy rubbish bins.
While the demonstrators seemed to have no single cause, many of them shouted the name of Mumia Abu-Jamal, who has been on death row since 1981 when he was convicted of shooting a police officer in the centre of Philadelphia.
In another part of Philadelphia, demonstrators favouring the execution of Mumia gathered at a rally addressed by Ms Maureen Faulkner, the widow of the murdered police officer. She said she would never abandon her family's goal of "getting closure", which is taken to mean the execution of Mumia, whose sentence is under appeal.
Mr Timoney has become a high-profile figure in Philadelphia since he became police commissioner in 1998 after resigning as deputy commissioner of the New York Police Department. He was born in the Liberties area of Dublin 52 years ago. His father was a tailor but also worked as a doorman.
He said he became a policeman on a whim when he took the exam along with friends in New York. He rose through the ranks to the number two spot and is credited, along with his chief, William Bratton, with reducing crime in the New York area.
Mr Timoney, who runs marathons and rows on the local Schuykill River, has spent the past year preparing for this convention and cancelled a trip to Dublin last May.