The complainants, almost all men who were aged between seven and 19 at the time of the alleged offences, have identified 84 people as responsible for attacks said to have taken place between 1963 and 1992.
Sixteen of the 84 alleged perpetrators have been named more than once, according to North Wales police, and six are believed still to be alive. One suspect has been arrested to date.
The scale of the alleged abuse was revealed on Monday in a National Crime Agency report on the progress of Operation Pallial, the criminal investigation ordered into north Wales care homes by David Cameron at the height of the sex abuse scandal last November.
Detective Superintendent Ian Mulcahey, one of the officers leading Operation Pallial, said many alleged victims had “provided graphic accounts of abuse, in some cases of very serious criminality”.
He said: “These are serious allegations that will be thoroughly investigated. We are prioritising our activities to ensure that any level of ongoing risk to the public is minimised.”
At a press conference in north Wales on Monday, detectives said the alleged offences ranged from serious physical assaults to rape.
They said that most of the 140 alleged victims had described a “clear element of grooming with a serious abuse of trust and dereliction of duty of care”.
They added in the Operation Pallial report: “Complainants have provided accounts of serious criminal offences committed against young and vulnerable people by adults charged with their care.”
Police said 75 of the 84 people suspected of abuse were male. Police expect to make more arrests within weeks as Operation Pallial enters “phase two”, with detectives building evidence against key suspects.
Previously, many of the complaints were believed to relate to the now-closed Bryn Estyn care home in Wrexham, but 18 institutions have now been linked to offences. Of the 140 alleged victims, 76 have made their first formal complaint since November.
Operation Pallial was sparked by the disastrous BBC Newsnight report in November last year that led to false child abuse allegations against the former Tory treasurer Lord McAlpine.
That Newsnight programme carried testimony from Steve Messham, a victim of abuse while in the north Wales care system, which later turned out to be mistaken. Messham also claimed that the Waterhouse inquiry that examined care in north Wales in 1996 did not properly investigate abuse allegations or the involvement of a paedophile ring.
On Monday, the detective leading Operation Pallial said he had found no evidence so far that the abusers acted together.
“We are still in the early stages of an investigation. We are looking at individual complaints at the moment and if evidence takes us there that tends to support allegations of collusion between other parties then we will investigate it thoroughly,” said Mulcahey.