LOS ANGELES  The low-budget alien movie “District 9” was No. 1 at the weekend box office with an estimated $37 million in ticket sales, a stronger than expected result fueled by a quirky marketing campaign.

“District 9,” an R-rated social satire about a spacecraft that stalls over Johannesburg, cost only $30 million to make. Peter Jackson of “Lord of the Rings” fame was the driving force as a producer, and the 29-year-old Neill Blomkamp, whose previous feature experience is almost entirely confined to visual-effects work, was the director.

Perhaps because of Mr. Blomkamp’s lack of directing experience and the film’s budget, Hollywood saw “District 9” mostly as an also-ran, a little fan-boy picture that was overlooked as the pack focused on behemoths like “G.I. Joe: The Rise of Cobra.”

But Sony Pictures Entertainment saw opportunity. The studio, which acquired distribution rights to the film, backed “District 9” with an unusual advertising campaign that echoed the paranoia of the movie’s story line. Billboards and bus ads, for instance, encouraged people to call a toll-free number to report nonhuman activity, resulting in thousands of calls.