LOS ANGELES - Movie-goers were easy prey for a double dose of space invaders.

The sci-fi smackdown "Alien vs. Predator," featuring the creatures of the "Alien" and "Predator" franchises, debuted as the No. 1 weekend movie with $38.25 million, studio estimates showed Sunday.

"Princess Diaries 2: Royal Engagement," with Anne Hathaway and Julie Andrews reprising their royal roles, opened in second place with a three-day gross of $23 million, pushing its total since premiering Wednesday to $37.2 million.

"Alien vs. Predator," featuring Sanaa Lathan and "Aliens" and "Alien 3" co-star Lance Henriksen, centers on a human expedition that stumbles on a space-monster blood feud in an ancient pyramid buried below Antarctica.

The movie debuted slightly ahead of the $36.4 million debut of last summer's horror hybrid "Freddy vs. Jason," which paired the slashers of the "Friday the 13th" and "A Nightmare on Elm Street" franchises.

"This continues the trend of combining franchises to very strong box-office results," said Paul Dergarabedian, president of box-office tracker Exhibitor Relations.

Audiences shelled out $16.8 million to see "Alien vs. Predator" on Friday, but the movie's gross fell to $12.5 million Saturday, a steep 26 percent decline. Most new movies do better business on Saturday than Friday.

That's a sign that "Alien vs. Predator" could follow the pattern of "Freddy vs. Jason" and other horror tales, which tend to open well then plunge in subsequent weekends.

Still, "Freddy vs. Jason" topped out at a healthy $82.2 million domestically. Budgeted at $60 million, "Alien vs. Predator" would turn a solid profit for 20th Century Fox if it matches the gross of "Freddy vs. Jason."

1. "Alien vs. Predator," $38.25 million.

2. "Princess Diaries 2: Royal Engagement," $23 million.

3. "Collateral," $16 million.

4. "Yu-Gi-Oh! The Movie," $9.4 million.

5. "The Bourne Supremacy," $8.3 million.

6. "The Village," $7 million.

7. "The Manchurian Candidate," $6 million.

8. "Little Black Book," $3.7 million.

9. "I, Robot," $3.6 million.

10. "Spider-Man 2," $3.4 million.