By Steve Gorman

LOS ANGELES (Reuters) - Big-screen Iron Man Robert Downey Jr. scored his second No. 1 movie of the summer on Sunday as Hollywood spoof "Tropic Thunder" ended the monthlong reign of "The Dark Knight" atop the North American box office.

"Tropic Thunder," a farcical combat movie within a comedy that also stars Ben Stiller and Jack Black, grossed $26 million during its first weekend in U.S. and Canadian theaters, bringing its five-day estimated total from Wednesday's opening to $37 million.

Downey who appears in the film in blackface, portraying a white actor playing a black action hero, also starred in the summer's first chart-topping movie, Marvel Studios' "Iron Man."

The superhero drama, which like "Tropic Thunder" was distributed by Viacom Inc's Paramount Pictures, grossed nearly $99 million its opening weekend in May.

In "Tropic Thunder," Downey, Stiller and Black star as self-absorbed Hollywood actors caught up in a real-life battle with narco-terrorists while filming a war movie in Southeast Asia. The film was directed, co-written and co-produced by Stiller.

The blockbuster Batman sequel "The Dark Knight" slipped to No. 2 with $16.8 million in ticket sales in its fifth weekend of release but broke yet another commercial barrier along the way by becoming the second-highest-grossing film ever, according to box office tracking service Media By Numbers.

"The Dark Knight," starring Christian Bale as Batman and the late Heath Ledger as the villainous Joker in his last completed role, has now amassed more than $471 million in domestic ticket sales. That tally ranks second only to the $601 million grossed by all-time champion "Titanic."

Adjusted for inflation, though, the 1997 blockbuster stands at No. 6 in the record books, far behind "Gone with the Wind" at $1.4 billion in today's dollars, according to tracking firm Box Office Mojo.

The Batman sequel surpassed the landmark sci-fi adventure "Star Wars" -- $461 million -- as No. 2 on the all-time box office list on Saturday. Distributor Warner Bros., a unit of Time Warner Inc, has said it expects "Dark Knight" to end up with about $520 million in domestic receipts.

The latest addition to the "Star Wars" franchise, a newly animated tale called "Star Wars: The Clone Wars," opened at No. 3 this weekend with an estimated $15.5 million in North American ticket sales.

The weekend's only other wide release, an American remake of the South Korean horror film "Mirrors," starring Kiefer Sutherland of the hit TV series "24," grossed $11.1 million to land at No. 4.

Last week's runner-up at the box office, the stoner comedy "Pineapple Express," fell to No. 5 with $10 million, a drop-off of nearly 60 percent from its opening tally.

Woody Allen's latest offering, the well-reviewed romantic comedy "Vicky Cristina Barcelona," starring Javier Bardem and Penelope Cruz, opened in just under 700 theaters nationwide to gross $3.7 million and round out this weekend's top 10.

Domestic box office receipts overall this weekend stood at $126 million, down 3 percent from the same weekend a year ago.© Copyright 2008 Reuters.