LOS ANGELES (Reuters) - A trio of animated secret agent guinea pigs broke the spell of the new Harry Potter movie to take the top spot at the North American box office, according to studio estimates issued on Sunday.

"G-Force," a Disney live action and computer-generated animation family feature, earned $32.1 million in the United States and Canada, pushing "Harry Potter and the Half-Blood Prince" down to second place with $30 million in its second weekend.

The sixth movie in the boy wizard franchise dropped 61 percent from its huge opening last weekend -- a fall-off that was in line with studio expectations -- but is still on course for becoming the second biggest Potter movie so far, distributors Warner Bros said.

Internationally, "Half-Blood Prince" raked in $84.4 million in 64 countries over the weekend to take its worldwide total so far to $627.1 million.

Dan Fellman, president of domestic distribution at Warner Bros, said the earnings in North America and Canada had been hampered by the movie's small presence so far in IMAX movie houses. IMAX roll-outs will expand in the coming week.

"Our IMAX support will pop in this week and it looks like the movie is headed north of $300 million (in North America) which will be the second biggest Harry Potter of all time on the domestic side," Fellman said. The biggest movie was the first one, 2001's "Harry Potter and the Sorcerer's Stone."

Delighted Disney executives said "G-Force," which features cute talking guinea pigs as action heroes, played well to family audiences and that more than half the box office had come from 3D showings.

"We are very, very pleased with the results when you are competing with movies like 'Harry Potter' and 'The Ugly Truth,'" said Marc Zoradi, president of Walt Disney Motion Pictures Group.

Paul Dergarabedian of Hollywood.com said that "G-Force" had benefited from a "great marketing campaign and a premise that had people scratching their heads while simultaneously grabbing the family."

Romantic comedy "The Ugly Truth," starring Katherine Heigl of TV medical show "Grey's Anatomy" and British heart-throb Gerard Butler, brought in a solid $27 million to take third place -- above expectations, distributor Sony Pictures said.

Rory Bruer, president of worldwide distribution for Sony Pictures, said the movie had cost just $38 million to make and had played particularly well to women over the age of 25.

"It all paid off. The chemistry between Heigl and Butler is tremendous," Bruer said.

The weekend's other strongest new showing was horror film "Orphan" which took fourth spot at the box office with $12.7 million.

The glut of new offerings pushed 20th Century Fox's "Ice Age: Dawn of the Dinosaurs" from second place to fifth. The animated movie has brought in $171.3 million domestically in four weeks.

Universal Pictures' "Bruno" tumbled another 67 percent in its third week of release with $2.7 million over the weekend, taking the total for British actor Sacha Baron Cohen's outrageous gay spoof to a modest $56.5 million.

Universal is a unit of General Electric Co's NBC Universal, and Fox is a unit of News Corp. Jill Serjeant