Details of the CIA's "enhanced interrogation" techniques, considered by many to be torture, are set to be revealed in a long-awaited report.
The Senate Intelligence Committee's report is expected to find that such interrogations failed to yield life-saving information.
The techniques were used on al-Qaeda terrorism suspects in the years following the 9/11 attacks on the US.
Security was increased at US facilities around the world ahead of publication.
Embassies and other sites were taking precautions amid "some indications" of "greater risk", a White House spokesman said.
The Senate committee's report runs to more than 6,000 pages, drawing on huge quantities of evidence, but it remains classified and only a 480-page summary will be released.
Analysis: Jon Sopel, BBC North America editor
What more can we learn about the CIA's interrogation programme from this heavily redacted report? Based on leaks, Tuesday's release seems to answer three major questions
- Were the interrogation methods - torture if you like - more extensive and more brutal than previously admitted? It looks like the conclusion is "Yes"
- Did these interrogation techniques deliver life-saving intelligence to the US? That answer appears to be "No"
- Were CIA officials at the time honest with the White House on what the programme was getting up to? Again, "No"
We can also expect the beginning of a counterblast of speeches, editorials and comments from those in charge of the CIA at the time attacking the Congressional report.
But White House officials - while supportive of the release in principle - nervously dispatched Secretary of State John Kerry to encourage the committee to think twice about releasing this report into a volatile world. That didn't work.
As well as detailing the controversial methods used by CIA operatives in an effort to extract information from high-value suspects, the report is expected to say harsh interrogations failed to deliver appropriate results.
Publication of the report has been delayed amid disagreements in Washington over what should be made public.
Senate Intelligence Committee chair, Dianne Feinstein, who initiated the investigation five years ago, was involved in months of discussion with the White House about what could be released.
She is expected to brief journalists as the findings are published on the committee's website.
President Barack Obama halted the CIA interrogation programme when he took office in 2009, and has acknowledged that the methods used to question al-Qaeda prisoners amounted to torture.
During the presidency of George W Bush, the CIA operation against al-Qaeda - known internally as the Rendition, Detention and Interrogation - saw as many as 100 suspected terrorists held in "black sites" outside the US.
They were interrogated using methods such as waterboarding, slapping, humiliation, exposure to cold, and sleep deprivation.
Leaks about the Senate report first emerged in August this year, prompting Mr Obama to declare: "We tortured some folks. We did some things that were contrary to our values."
The US president added that he believed officials at the time had used harsh methods because of the "enormous pressure" to prevent another attack on the US in the wake of 9/11.
A previous investigation into the programme, by the US justice department, ended with no criminal charges in 2012 - a result that angered civil rights organisations.
Reports that US intelligence had used waterboarding first emerged in the years following the 9/11 attacks, and the CIA confirmed in 2008 that it had interrogated three al-Qaeda suspects using that method in 2002 and 2003.
'Enhanced interrogation': Key facts
When did it happen? In the years following the 9/11 attacks in 2001 at "black sites" in a range of countries as US intelligence raced to find the perpetrators.
What were the main methods? Sleep deprivation, confinement in small spaces, slapping, humiliation, waterboarding - simulated drowning.
Who was waterboarded? Khalid Sheikh Mohammed, alleged 9/11 mastermind; Abu Zubaydah, al-Qaeda's "travel agent"; Abd al-Rahim al-Nashiri, alleged mastermind of 2000 attack on USS Cole.
Why is report released now? Head of the Senate Intelligence Committee Dianne Feinstein began investigation in 2009, but report's publication was held up by negotiations with CIA over how much could be released.
White House spokesman Josh Earnest told reporters on Monday that the Obama administration welcomed the impending release, but said there were "some indications" it could increase the risk to US facilities across the world.
"The administration has taken the prudent steps to ensure that the proper security precautions are in place." Mr Earnest said.
Human Rights Watch said last-minute attempts to delay publication showed how important the document was for understanding the "CIA's horrific torture programme".
"US foreign policy is better served by coming clean about US abuses rather than continuing to bury the truth," HRW's Washington director Sarah Margon said.
There have been mixed reactions from US allies to speculation about the report, with Poland saying it hoped it would "not have a negative impact on relations".
Poland has been criticised by the European Court of Human Rights for hosting secret CIA prisons, known as "black sites".
But a senior British MP said the UK had repeatedly denied any wrongdoing and the report would not expose any culpability.
"My basic instinct is that when this report comes out, Britain is not going to be seen in any embarrassing light," Foreign Affairs Select Committee Chairman Richard Ottaway told the BBC.
Despite reports that CIA operatives went beyond legal interrogation limits imposed by the Bush administration, the former president has led the charge against the report's release, defending the CIA on US TV.
"We're fortunate to have men and women who work hard at the CIA serving on our behalf," he told CNN on Sunday.
Others have joined Mr Bush to dismiss the as-yet unreleased report, including reports it will say the CIA misled key members of the Bush administration about the programme.