(USA Today) -- One of the cheapest movies of the year scored the biggest box-office upset of summer as The Purge knocked Fast & Furious 6 from the top spot and trounced The Internship.
Purge, a $3 million horror film that was expected to vie for third place this weekend, raked in $36.4 million, according to studio estimates from box-office trackers Hollywood.com.
The debut stunned analysts, who forecast $17 million for the critically ravaged Ethan Hawke movie, expected to be disposable fare between studio tent-poles.
But the film, a futuristic tale where all crime is legal for one night a year, began picking up online momentum last month, when its trailer - shot largely on shaky, hand-held cameras - hit the Internet.
The debut marked the latest low-budget horror movie to upend major studio films. In January 2012, the $1 million horror film The Devil Inside opened to $34 million on its way to $53 million. In January, the $15 million fright flick Mama took No. 1 with a $28 million debut that led to a $72 million haul.
But Purge pulled the upset in summer, when the industry's most expensive movies dominate the cineplex.
"The Purge is in a bulletproof genre and its unique concept brought out viewers tired of franchise film after franchise film," says Tim Briody of Boxofficeprophets.com.
Ray Subers of Box Office Mojo says the film resonated with younger audiences with a savvy marketing campaign that included fake government warnings about the approaching night when all crime would be legal and emergency services would be suspended for 12 hours.
"Universal Pictures has done an excellent job (with) eye-catching posters and billboards that at first glance appear to be a public service announcement," he says.
Critics attempted their own public service in warning moviegoers away from the movie, which earned a thumbs-up from just 41% of critics, says survey site Rottentomatoes.com.
Even moviegoers seemed only mildly entertained, as just 59% of audiences liked it, the site says, while other moviegoers gave it just a "C" grade, according to CinemaScore.
But the concept was enough to knock Furious from the perch it held for two weeks. The Vin Diesel action film did $19.8 million, good for second place. Now You See Me, the film about bank-robbing magicians, was third with $19.5 million.
The Internship, the $58 million comedy starring Vince Vaughn and Owen Wilson, found little to laugh about this weekend. The movie took fourth place with $18.1 million, which met forecasters' modest projections but was no match for its micro-budget competition.
The animated fantasy Epic rounded out the top five with $12.1 million. Final figures are expected Monday.
Attendance and ticket sales are down about 6% from the same period last year, says Hollywood.com.