'The Dark Knight' on Sunday became the second movie in Hollywood history to top US$500 million (S$708 million) at the domestic box office, raising its total to US$502.4 million, according to estimates from distributor Warner Bros.

The film hit that mark in just over six weeks, half the time it took Titanic, which reached US$500 million in a little more than three months. Titanic, the biggest modern blockbuster, remains No. 1 on the domestic charts with US$600.8 million.

Despite its brisk pace, The Dark Knight is not expected to approach the total for Titanic, which put up smaller numbers week after week but lingered at the top of the box office for months.

Dan Fellman, head of distribution at Warner Bros, said he expects The Dark Knight to finish at about US$530 million, though it could reach US$550 million if business persists as strongly as it has.

'I keep raising the number because it just keeps holding better than expected,' Fellman said.

The Dark Knight will climb to about US$505 million by the Labor Day holiday on Monday, the conclusion of Hollywood's busy summer season. That amounts to nearly one-eighth of Hollywood's overall summer revenue of US$4.2 billion, which edges the previous summer record of US$4.18 billion set last year, according to box-office tracker Media By Numbers.

Factoring in today's higher admission prices, The Dark Knight would need to take in about US$900 million to match the number of tickets sold by Titanic.

Labor Day weekend was generally sleepy at theatres, with a rush of new movies failing to find much favour with audiences. Through Sunday, Paramount's comedy Tropic Thunder remained No. 1 for the third straight weekend with US$11.5 million.

The 20th Century Fox sci-fi thriller Babylon A.D. with Vin Diesel debuted in second place with US$9.7 million, while Overture Films' espionage drama Traitor, starring Don Cheadle, opened at No. 5 with US$7.9 million. -- AP