Stiller comedy beats 'Terminator' at box office

* "Night at the Museum" sequel beats forecasts

* "Terminator" sequel falls short without Schwarzenegger

LOS ANGELES, May 25 (Reuters) - Ben Stiller's "Night at the Museum" sequel crushed the fourth installment of the "Terminator" franchise in a battle for North American box office supremacy between two special-effects extravaganzas.

According to studio estimates issued on Monday, "Night at the Museum: Battle of the Smithsonian" earned $70 million during the four days beginning Friday, outperforming the expectations of its distributor, 20th Century Fox.

On the other hand, Warner Bros.' "Terminator Salvation" -- the first in the series without Arnold Schwarzenegger -- made its debut at No. 2 with a disappointing $53.8 million.

"Star Trek" slipped one place to No. 3 with $29.4 million in its third weekend. Reigning champ "Angels & Demons" fell to No. 4 with $27.7 million in its second weekend, although the Tom Hanks Vatican thriller remained the top choice internationally.

Big studio movies rarely open against each other during the lucrative summer period in order to avoid cannibalization. But in this case families made up almost half of the audience for "Night at the Museum" while men accounted for 70 percent of the audience for "Terminator."

Stiller, one of Hollywood's most reliable box office performers, set a personal live-action record with his new film. His previous best was the $46 million start for "Meet the Fockers" in 2004.

The 43-year-old actor reprises his role as a night security guard at a museum where historical figures come to life. The cast also includes Robin Williams as former U.S. President Theodore Roosevelt and Amy Adams as aviatrix Amelia Earhart.