The new romantic drama Dear John unexpectedly ended the seven-week reign of Avatar at the weekend box office in North America on Sunday, pulling in large numbers of young female moviegoers.

Dear John grossed an estimated $32.4m in the three-day period since opening Friday, said distributor Screen Gems, the low-budget division of Sony Corp.

It had hoped for an opening of about $20m on a weekend when many Americans forsake movies to watch the Super Bowl, traditionally the year's most-watched television broadcast. The football championship starts around 23:25 GMT in Miami.

Dear John stars Channing Tatum and Amanda Siegfried as lovers whose romance is curtailed by the September 11 attacks. It is directed by Swedish film maker Lasse Hallstrom and based on The Notebook, a novel by Nicholas Sparks.

Audiences for the film were 84% female and two-thirds were under the age of 21, Sony said.

The last romantic comedy to hit theatres was the widely panned Leap Year about a month ago. The Lovely Bones, another box office disappointment, also has made a play for young women after failing to get any traction during the annual movie awards season.

Avatar earned $23.6m in its eighth weekend, taking its total to $630.1m. James Cameron's sci-fi blockbuster surpassed the $601m haul of his 1997 release Titanic last Tuesday to become the biggest movie of all time in the United States and Canada.

The data are not adjusted for ticket-price inflation or for the higher cost of 3D engagements. The film was released by 20th Century Fox, a unit of News Corp.