The state parole board has been flooded with more than 367,000 online letters protesting the release of cop-killers since the panel voted to free police assassin Herman Bell, the PBA said Monday.

Bell — who helped ambush and kill two officers in 1971 — was slated to be freed Tuesday due to the board’s controversial decision last month.

But his release was put on ice when the Patrolmen’s Benevolent Association sued and a judge ordered Bell kept behind bars until he makes a ruling.

PBA President Pat Lynch said his union launched a week-long radio-commercial campaign after the board voted to release the 70-year-old Bell, prompting the deluge of letters.

Users of the PBA’s Web site can click on the “Keep Cop-Killers in Jail” icon on its homepage, which allows them to send a letter opposing parole for all convicted killers of NYPD cops.

“We brought our case for justice and our call for help to keep cop-killers in jail to the public, who responded with overwhelming support,” Lynch said in a statement.

“Parole may be appropriate for some low-level criminals, but it is never appropriate for cop-killers,” he added. “We are deeply grateful to all of those good people who took the time to visit the PBA’s Web site in order to object to the release of cop-killers onto our neighborhood’s streets.”

Bell, a former Black Liberation Army member and domestic terrorist, has served more than 40 years in prison for the slayings of NYPD Officers Joseph Piagentini and Waverly Jones.

He and two other Black Liberation Army members lured Piagentini, 28, and Jones, 33, to a Harlem housing project with a bogus 911 call on May 21, 1971. When the unsuspecting cops approached, the three men ambushed them and opened fire, killing them.

Bell was sentenced to 25 years to life behind bars for the slayings.

He later pleaded guilty to manslaughter in the BLA-orchestrated death of a third cop, Sgt. John Young, in San Francisco.

The PBA said that at one point during its campaign to keep Bell behind bars, the parole board received more than 52,000 letters a day protesting his release.

“It should be clear to any reasonable person that no one believes that cold-blooded cop-killers like Herman Bell should ever be released from prison,’’ Lynch said.

The union filed a lawsuit against the parole board, arguing the board ignored prosecutors’ statements at Bell’s 1975 trial in which they said he was “beyond redemption and can never be rehabilitated.’’

Bell remains held upstate in the maximum-security Shawangunk Correctional Facility.

State Supreme Court Justice Richard Koweek is reviewing the case. He said he’ll be making a ruling “in the near future.”