NEW YORK (AFP) - A new study spotlights America's brutal legacy of racial violence, revealing that nearly 4,000 blacks were lynched in the south from 1877 to 1950, an average of more than one a week for 73 years.

The Equal Justice Initiative, a human rights group in Alabama that conducted the research, said present day racial discrimination and criminal justice problems are rooted in the country's violent past.

"In Germany you're forced to deal with the legacy of the Holocaust," the organisation's founder Bryan Stevenson said.

"We do the opposite in America. We haven't committed ourselves to truth and reconciliation, we haven't really tried to deal with the consequences of this legacy," he said.

In years of research, his organisation documented 3,959 lynchings in 12 states of the South from 1877 to 1950, with the majority of the killings taking place between 1880 and 1940.

The Justice Initiative says it found 700 more victims than any previous study into the thousands of black men, women and children who were lynched in the South in this period.

To escape constant threats nearly six million black Americans fled the South from 1910 to 1970, pouring into urban ghettos in the northern and western United States, the report said.

Stevenson said around 20 per cent of lynchings were astonishingly public spectacles, watched by hundreds if not thousands of white Americans, including elected officials.

"I think that most people aren't very well informed about this era," Stevenson said. "It's so grotesque to imagine that people in the community not 100 years ago would engage in this kind of barbarism."

Spectators munched on picnics, and sipped lemonade and whiskey as the victims were tortured, mutilated and dismembered, their body parts distributed as souvenirs, the report found.

In 1904, for example, Luther Holbert was accused of killing a white landowner and lynched with a black woman believed to be his wife in Doddsville, Mississippi before hundreds.

Both victims were tied to a tree while members of the mob chopped off their fingers and distributed them, the report said.

Their ears were sliced off and Holbert was beaten so badly that his skull was fractured. The mob then used a large corkscrew to bore holes and pull out chunks of flesh from the victims' bodies.

The Initiative said hundreds of victims were murdered for trying to vote or improve their status, or more minor infractions such as refusing to step off the sidewalk or bumping into a white woman.

Not a single white person was convicted of murder for lynching a black person in America during this period, the report said.

Blacks are still disproportionately more likely to be arrested, convicted, jailed and sentenced to death in the United States.

Nearly 42 per cent of those currently on death row are black and blacks make up 34 per cent of those executed in the United States since 1976, although blacks make up only 13 per cent of the population.

Stevenson connected the lynchings to problems in America's criminal justice system today and to a disproportionate number of blacks who are killed at the hands of police officers.

"Lynching led to the death penalty, which led to mass incarceration, which dehumanised and criminalised people of colour disproportionately," he said.

The slogan "black lives matter" shouted at recent protests across the United States in response to killings of unarmed black men by police is rooted in the same history, he said.

"I don't think many Americans appreciate there was an era when terrorism was widespread and committed by Americans against other Americans," said Stevenson.

The Justice Initiative now intends to raise money and erect monuments to honour the victims, a move which Stevenson expects to spark controversy in at least some communities.

While there are few public markers to lynchings, dozens of memorials honour architects of racial subordination and political leaders who believed in white supremacy, the report said.

The 12 states subject to the report are Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, Mississippi, North Carolina, South Carolina, Tennessee, Texas and Virginia.