Among its 480 pages are descriptions of captives of the US war on terrorism being threatened with sexual assault, of a man being threatened with a buzzing drill. There are, it has been reported, descriptions not just of something the world came to know as "water-boarding", but of prisoners having their heads thrust into baths of water "to the point of death" as a doctor stood by to revive them should they slip away.

"This was real torture," an unnamed source with knowledge of the material told London's Telegraph newspaper as far back as September.

As the US prepares for the release of declassified 480-page executive summary of the 6300-page report, US embassies around the world stepped up security for fear of it provoking violence.

The report is the result of an investigation by the Democratic Party-dominated Senate Intelligence Committee, based on a review of millions of pages of classified CIA documents.

It is expected to allege that the CIA's use of what it calls "enhanced interrogation techniques" was far more extensive than previously acknowledged, that the CIA exceeded the already broad parameters for the interrogations set by Department of Justice after the attacks on September 11, 2001, and that the agency later exaggerated the extent and quality of the intelligence it gathered to justify the program.

For months the Committee, headed by leading Democratic senator Dianne Feinstein, has haggled over the redaction process with the CIA, the White House and the State Department , delaying the document's release.

Earlier this year the CIA was  forced to admit that not only had it illegally spied upon the Senate committee's staff as its investigation continued, but that it had also lied about that activity. The CIA's director John Brennan issued an extraordinary apology for its illegal domestic spying in July.

Political fighting over the report's content and release has been going on all year and has intensified over the past few days.

The man in charge of the CIA interrogation program, Jose Rodriguez, a former director of the National Clandestine Service of the CIA, wrote in a Washington Post opinion piece published on December 5: "The interrogation program was authorised by the highest levels of the US government, judged legal by the Justice Department and proved effective by any reasonable standard.

"The leaders of the Senate and House Intelligence Committees and of both parties in Congress were briefed on the program more than 40 times between 2002 and 2009. But Representative Nancy Pelosi tried to deny that she was told in 2002 that detainees had been water-boarded," he wrote.

"That is simply not true. I was among those who briefed her."

The Republican chair of the House Intelligence Committee, Mike Rogers, said the report's release would constitute a public relations victory for terrorist groups.

"They don't have to be accurate or right. They just have to believe it's true and they will take advantage of that. We know that ISIL propaganda operations will - this is the motherlode for them."

Without addressing the specific allegations, former president George W. Bush, who ordered the program, offered his support for those involved in it. .

"We're fortunate to have men and women who work hard at the CIA serving on our behalf," he told CNN. "These are patriots and whatever the report says, if it diminishes their contributions to our country, it is way off base."

Former CIA deputy director John McConnell also spoke out against the report's release. "They've known of this potential damage for months, so it's stunning that something would be coming to fore at this point," he said..

"It is blindingly obvious that putting this report out will not only cause trouble for American diplomacy and put Americans in danger, not because it's true and accurate but because of the way it's written and what it alleges."

But Senator John McCain, the senior Republican who was tortured in Vietnam, has long disagreed with those who have supported the program. "When you torture someone they will say anything you want to hear to make the pain stop. So I never, ever believed this bologna that, well, because of water-boarding they got information," he said in April this year. "There's a couple stories [in the report] that are so chilling that I can't repeat them right now".

Democratic senator Claire McCaskill told CBS the release of the report was essential.

"It exposes what the world already knows and that is that the United States engaged in torture, but my feeling . . . is that this is a gut-check moment for our democracy," she said.

"This report would never happen in North Korea, or China or Russia."

A spokesman for another Democratic member of the committee, Mark Udall, told Politico: "Senator Udall remains committed to getting the truth out about the CIA's misguided, brutal and ineffective detention and interrogation program.

"His belief has not wavered. The Senate Intelligence Committee and the administration have arrived at a mutually agreed-upon set of redactions that protect national security while also ensuring that the truth comes out about this dark chapter in American history.

"Our nation has proven time and again that we can and should responsibly acknowledge our mistakes - even when the United States is engaged in military activities abroad, as we were in Iraq when the US Army publicly released its investigation into Abu Ghraib - and that doing so makes us stronger and more secure."

When elected, Mr Obama ordered that enhanced interrogation be stopped, but his administration has not pursued any criminal investigations or sanctions against people involved in the program.