Directed by James Cameron, who made Terminator and Titanic, Avatar uses 21st-century 3D technology to tell the age-old story of giant blue boy meets giant blue girl on a distant planet.

Costing upwards of £180 million, Avatar mirrors Titanic in that it represents a reputation-on-the line moment for Hollywood’s ultimate showman.

Speaking before the premiere, Cameron said he was delighted to finally unveil the picture given the “huge buzz” that has surrounded it.

“I’m so relieved that we can hold our head high, that we got the picture done on time. It’s been a four-and-half-year process, so tonight we’ll pull the cover back and show the world.”

Sigourney Weaver, who worked with Cameron on Aliens, plays a scientist toiling on the planet of Pandora, home to the Na’vi, a community of tall, blue beings.

Weaver said she jumped at the chance to be part of the “adventure” of Avatar.

“I always know with Jim that you are never in better hands, there’s no-one who is going to fight harder, stay longer, work harder to give the audience a helluva experience.”

Avatar uses 3D “performance capture” technology to blend the real performances of the actors with their computer-generated screen counterparts.

Sam Worthington, who plays a former US Marine trying to find out more about the Na’vi culture, said his initial concern that the technology would get in the way of the actors’ performances proved unfounded.

“I try to be a very subtle actor and I was worried that the nuances of the performance wouldn’t translate via the bits and bytes of the computer. But in my opinion that is 100% my performance, every glimmer in my eye, every smirk, every walk.”

Far from putting a barrier between actor and director, Cameron said performance capture led to “probably the best director-actor relationship I’ve ever been involved in. In the virtual working process I’m not distracted by the lighting, the time of day, the sun setting .... a thousand questions that pull a director’s mind away from the process of working with the actors”.

Avatar is set in 2154, a time when Earth is in the grip of an energy crisis and must find new sources of power to survive. Standing in the way of a mega corporation getting its hands on minerals are the Na’vi.

“The story is by design classic in its broad strokes, but we have plenty of twists and turns in store for the audience,” said Cameron.

“I’ve dreamed of creating a film like this, set on another world of great danger and beauty, since I was a kid reading pulp science-fiction and comic books by the truckload, and sitting in math class drawing creatures and aliens behind my propped-up textbook.”

Of the picture’s anti-war, pro-environment messages, he said: “Obviously there’s a connection to recent events, and there was a conscious attempt to evoke the Vietnam era imagery with the way guys jump off the helicopters and so on.” The story also echoes the way Europeans took over the Americas, he said.

“There’s this long history of the human race written in blood that goes back to the Roman Empire and further back, where we have this tendency to just take what we want without asking.”

Humans also have a misguided sense of entitlement about the natural world, he said. “We’re going to find out the hard way if we don’t wise up and start seeking a life that’s in balance with the natural cycles of life on Earth.”

Of the claims being made online that the film will revolutionise cinema in the same way as the talkies, Cameron said simply: “We’ll see.”

With so much spent on inventing the technology to make Avatar, there has been speculation that Cameron has in mind more than just the movie that premiered last night. Though he already has in mind stories for two more films, Cameron is taking nothing for granted.

“That will only happen if we make some money with the first one,” he said.