Going Clear: Scientology and the Prison of Belief premiered Sunday at the Sundance Film Festival to a packed house — not with a star-studded red carpet, but with police protection.

A week before the premiere, the Church of Scientology took out full-page ads in the New York Times and Los Angeles Times claiming the documentary is filled with falsehoods.

Based on Lawrence Wright's 2013 book of the same name, Oscar winner Alex Gibney's film claims that the church routinely intimidates, manipulates and even tortures its members, tracing the rise of the religion and its founder, former science-fiction writer L. Ron Hubbard, and his successor as head of the church, David Miscavige. Gibney also interviewed several former Scientology believers, including past executives.

Canadian director speaks out

Canadian director Paul Haggis, who appears in the documentary, has made headlines for speaking out against the Church of Scientology. (Michael Buckner/Getty Images) Canadian Paul Haggis, director of the Oscar-winning Crash, left the church in 2009 after decades of membership.

"I was really stupid. I was part of this for 30 years before I spoke out," he says in the film. "I was deeply ashamed."

As Haggis climbed "the bridge" to the most enlightened levels of Scientology, he finally learned Hubbard's ultimate theory: That a tyrannical galactic overlord named Xenu dropped frozen bodies from millions of years ago into volcanoes, and those spirits attach themselves to modern people today. Scientology is the means of ridding the body and mind of those spirits to become "clear."

VIDEO |​ Paul Haggis on his interest in Scientology and defending his friends

Another former member who left the church in 2013 said its approach is "like brainwashing."

Former church members followed

Several former church members featured in the film attended Sunday's premiere. Two said they were followed to Utah by Scientology investigators who photographed them at the Salt Lake City airport.

Going Clear filmmaker Alex Gibney poses for a portrait during the 2015 Sundance Film Festival on Monday. (Larry Busacca/Getty Images) The Church of Scientology released a statement Sunday that characterizes these former believers as "the usual collection of obsessive, disgruntled former Church members kicked out as long as 30 years ago for malfeasance, who have a documented history of making up lies about the church for money."

The church says Gibney refused to meet with the 25 members it offered as sources. Gibney says the church declined all requests for interviews, as did Miscavige and movie stars John Travolta and Tom Cruise, both of whom are Scientologists.

Tom Cruise and Nicole Kidman's split

Actors Nicole Kidman and Tom Cruise married in 1990 and divorced in 2001. Going Clear suggests Scientologists help break up their marriage. The film traces Cruise's relationship with the church, and claims it intentionally broke up his marriage with Nicole Kidman because she was not a believer. Scientology's biggest celebrity spokesman was largely absent from the church during his nearly 10-year marriage to Kidman.

4 takeaways from Going Clear, as observed in reviews of the film:

The documentary alleges that church leaders worked to "facilitate the break-up" of Kidman and Cruise by, among other things, making Cruise feel paranoid about his relationship, so much so that he ordered a tap on Kidman's phone.

The film claims church members worked to turn the couple's children, Connor and Isabella, against their mother in an attempt to sway decisions about custody.

Going Clear also alleges that the church tried to set Cruise up with future Homeland actress Nazanin Boniadi and even tried to give her a makeover to make her more attractive to the newly divorced actor.

The film also suggests that the church is holding sensitive information about John Travola's sexual orientation over the Saturday Night Fever star's head in an attempt to ensure his loyalty to Scientology

From left to right: Actors Michael Pena, Erika Christensen, Kelly Preston and John Travolta attend the Church of Scientology Celebrity Centre 45th anniversary gala in August. (Kevin Winter/Getty Images)

Degradation and torture

Members deemed to have somehow erred against the church were subject to degradation and torture, according to the film. They were deprived of sleep, fed scraps and forced to do hard labour. Sometimes they were beaten. One man was required to mop a bathroom floor with his tongue, according to the film.

Gibney and Wright said the church has threatened them with litigation. Former members have said they have fared far worse: they've been slandered online, followed, filmed and seen their loved ones stalked and intimidated.

Former Scientology spokesman and senior executive Mike Rinder said he hopes the film will raise public awareness about the church's methods.

"I would love it if the FBI, after seeing this film, said, 'We need to do something more energetic.'"

The Sundance Film Festival continues through Feb. 1.