“Transformers: Dark of the Moon” is poised for worldwide box office fireworks, with a feasible $400 million-plus launch for the long weekend.

After its aggressive 3D push Tuesday night, Paramount folds in today the remaining nationwide 2D locations for “Dark of the Moon,” screening the pic at a total 4,011 domestic locations over the Fourth of July frame.

“Transformers” is expected to see a colossal start worldwide as nearly every major overseas market, except China and Japan, will debut the pic this weekend.

B.O. pundits are predicting the tentpole to start out domestically at north of $150 million for six days, added to an international take that could rival the $260.4 million overseas benchmark set by Disney’s “Pirates of the Caribbean: On Stranger Tides.”

With the Fourth falling on a Monday, the weekend could stretch into a moviegoing marathon in the U.S.

Two Friday openers, Universal’s Julia Roberts-Tom Hanks starrer “Larry Crowne” and 20th Century Fox’s tween/teen pic “Monte Carlo,” provide holiday counterprogramming for domestic auds. “Larry Crowne” may have a slight edge given its skew toward adults over 35; Fox is predicting high single digits for “Monte Carlo,” which could split the teen girl demo with “Transformers” thanks to TV favorites Selena Gomez, Leighton Meester and Katie Cassidy.

Last year, Summit’s “The Twilight Saga: Eclipse” was the big holiday winner, with $176 million in six days (that pic also bowed on a Wednesday). “Transformers” could collect similar totals, but the 2010 frame also had help from “The Last Airbender,” with $69.3 million in five days.

Though “Transformers” won’t be able to sustain by itself another weekend-to-weekend improvement over the same frame last year, holdovers led by Disney/Pixar’s “Cars 2” will provide some much-needed support.

While overall box office, clicking behind 2010 by almost 7%, continues to gain steam, the big question this weekend will be whether “Transformers” will be able to reverse shrinkage in the 3D market. In the span of just a few months, 3D opening shares have dropped from the 60%-70% range to a more recent norm of around 45%. Last weekend, “Cars 2” hit an industry low for 3D at just 40%, according to Disney.

Par’s marketing has focused on selling “Transformers” as a 3D event, structuring some of the pic’s TV spots to highlight the format. But the exclusive 3D run elevated Paramount’s efforts to a whole new dimension. “You have a group of people who like seeing movies in 3D,” said Par vice chairman Rob Moore. “The challenge is to convince that other 20% of people who sometimes see movies in 3D that this one is worth it.”

Par is unlikely to be disappointed with “Transformers”: a domestic launch of $150 million at the low end, up to the $200 million foreseen by more optimistic observers would be more than suitable for the pic reportedly budgeted at $195 million.

Par’s using “Iron Man 2” for a comparable opening instead of “Transformers: Revenge of the Fallen.” The former launched Memorial Day weekend 2010 with $152 million in six days; “Revenge of the Fallen” tallied a six-day gross of $215 million after debuting on Wednesday, June 24, 2009.

Meanwhile, modest budgets for U’s “Larry Crowne” and Fox’s “Monte Carlo” means those studios aren’t risking much. “Monte Carlo” cost $20 million; “Larry Crowne’s” $30 million budget was fully financed by Vendome Pictures.

Among the weekend’s top repeat players, “Cars 2” should play best, followed by Sony’s “Bad Teacher” and then Warner Bros.’ “Green Lantern.” Sony’s R-rated comedy doubled what “Green Lantern” made on Monday, with $4 million vs. the comicbook pic’s $2.1 million take.