The 22nd film in Marvel Studios' saga landed on more screens than any movie ever has in US and Canadian theatres. And even then, the 4600 theatres the Walt Disney Co. has lined up may still not be able to keep up with demand.

From Thursday night, many theatres have been open round-the-clock. Seventeen AMC Theatres won't close for 72 hours straight. About $US120 million ($A171 million) in presales have already set records on advance ticketing services Fandango and Atom. AMC's website was crashed by early Endgame ticket buyers.

"It looks like we've gotten Thanos's snap," AMC said.

Just how massive the ticket sales will be by the end of Sunday has been one of the one of the industry's favorite guessing games. Can it clear $US300 million ($428 million) domestically? Is a $US1 billion worldwide weekend possible? Will Avengers: Endgame eventually rival the $US2.8 billion total gross of Avatar in 2009?

Regardless, records will fall - and they have already started to. Disney said on Wednesday that Avengers: Endgame grossed about $153 million in China on Wednesday, where it first opened. That's already the most lucrative single day ever in Chinese theatres. Only Star Wars: The Force Awakens had a larger single day gross, and its one-day $242 million haul came from both U.S. and Canada theatres.

"This is a seismic box-office event," said Paul Dergarabedian, senior media analyst for Comscore. "This is like the 100-year flood of movies." The current opening weekend record is held by the last Avengers movie - the 2018 preamble to Endgame, Infinity War. It debuted with $368 million domestically and $913 million worldwide.

Both of those records are likely toast. The worldwide haul is certain to be obliterated because Infinity War didn't debut in China until two weeks later. Endgame is opening worldwide more or less simultaneously everywhere except Russia.

Estimates range from $370 million to $428 million domestically, and between $800 million and $1.6 billion globally.

Endgame concludes not just the arc of the Avengers movies but signals the completion of the 22-movie Marvel Cinematic Universe, as mapped out by Marvel Studios President Kevin Feige.

Feige has been tight-lipped about what the next phase will be, though he has new reinforcements. Disney's acquisition of 20th Century Fox brings Marvel's X-Men and Deadpool under the same roof. Endgame will give a much-needed jolt to the box office, which is running about 16 per cent behind 2018. Ironically, though, the weekend won't be much an improvement over the same time frame last year since that's when Infinity War opened. And no other wide release is daring to open against Endgame.

Ultimately, the only thing standing in the way of Endgame is Endgame itself. With its running time of 181 minutes, theatres won't be able to fit as many screenings in per day as they'd like to.

"It looks like the demand is going to outstrip the supply but theatres are doing their best to keep up," said Dergarabedian. "There's only 24 hours in a day and it's a 3 hour and one minute movie."

Avengers: Endgame is now in cinemas.

- AP