LOS ANGELES — In the opening scenes of “Prince of Persia: The Sands of Time,” a wise man cautions the title character, played by Jake Gyllenhaal, not to take on too much. The warning: “You’re just not ready for this.”

The words proved an omen for Mr. Gyllenhaal in real life over the weekend, as an effort to recast him as an action hero — a star capable of anchoring a big summer movie — ended in disappointment. “Prince of Persia” sold an estimated $37.8 million in tickets in North American theaters over the holiday weekend, a weak performance for a film that cost about $200 million to make and carried global marketing costs of more than $100 million.

“Prince of Persia,” from the producer Jerry Bruckheimer and Walt Disney Studios, entered the market in second place. DreamWorks Animation’s “Shrek Forever After” was No. 1 in its second week, with a strong $55.7 million, an indication that positive word of mouth has followed the film’s so-so box office start. Premium-priced 3-D tickets also helped. Total domestic sales for “Shrek Forever After” now stand at $145.5 million.

The other new release of note, “Sex and the City 2,” was third, with about $37.1 million, according to Hollywood.com, which compiles box office statistics. Warner Brothers released this R-rated sequel, which cost about $100 million to make, on Thursday to grab any fans who planned to travel over the holiday. Adding in Thursday sales, “Sex and the City 2” sold a total of $51.4 million.