By Omar Fahmy and Ahmed Aboulenein

CAIRO (Reuters) - A riot erupted in a Cairo suburb on Tuesday after a policeman shot three people in an argument, killing one of them, security sources said, the latest incident to rouse public anger over alleged police brutality in Egypt.

Witnesses said the argument was over the price of a cup of tea. A video shot by a person at the scene and sent to Reuters showed a man lying still on the floor surrounded by angry onlookers. It was not immediately clear if he was dead.

One of the onlookers held up a bullet casing and accused the police of killing "poor" Egyptians.

A crowd quickly gathered, overturning a police vehicle and beating up another policeman at the scene, said a witness, who did not see the shooting but said he arrived at the scene in the well-to-do neighbourhood of Rehab shortly afterwards.

Rights activists say police brutality is widespread in Egypt and that there is a culture of impunity. The Interior Ministry says abuses are isolated and incidents are investigated.

"There are clashes between the police and locals. Security forces brought in two riot police vehicles and an armoured truck and the victim's family is here and pelting them with rocks," said the witness, who declined to be named for fear of reprisal.

"Security forces are retreating and promising justice but the crowd is demanding police hand over the killer."

Security sources said the argument involved three policeman and three civilians. The witness said two of the policemen had fled but the third was set upon by the crowd before being taken away by police.

"The interior ministry are thugs," chanted the crowd in a video sent by the witness.

Anger over perceived police excesses helped fuel the 2011 uprising that ended President Hosni Mubarak's 30-year rule and began on a Police Day holiday. Since then, police have regained their power and human rights groups say they have returned to their old ways.

Public anger against police has surged again in recent months.

In February, a policeman shot dead a driver in the street in an argument over a fare, prompting hundreds to protest outside the Cairo security directorate. There were also riots in Ismailia and the southern city Luxor over the authorities' handling of at least three deaths in police custody in a single week in November.

Egyptian security forces have also faced scrutiny over the killing of Italian researcher Giulio Regeni in Cairo this year. Human rights groups say his death bears the hallmarks of Egyptian security agencies. Officials deny involvement.

"A hundred years of isolated incidents," wrote one commentator on Twitter.

(Additional reporting by Ahmed Mohamed Hassan)