Media playback is unsupported on your device Media caption Watch clips from the show and interviews with the cast during its making

Doctor Who fans around the world should expect an "emotional wallop" at the end of the 50th anniversary episode, says the show's boss Steven Moffat.

"It's the most ambitious episode we've ever done," he said.

The episode, The Day of the Doctor, will be broadcast in more than 90 countries at the same time as it airs on BBC One on Saturday night.

The BBC says it is likely to be the largest simulcast of a TV drama in history.

"This event means it is a worldwide show not simply a British phenomenon," Moffat said.

The episode will also be screened in 3D in more than 1,500 cinemas across the world, including Australia, Russia, Brazil, Mexico, Ecuador, Spain, Sweden, Norway and Iceland.

The 50th anniversary adventure stars Matt Smith, David Tennant and John Hurt as different incarnations of the Doctor.

John Hurt's version of Doctor was first introduced at the end of the episode, The Name of the Doctor, broadcast in May.

Jenna Coleman plays companion Clara, while Billie Piper returns as Rose.

The episode will also feature the Daleks and the return of shape-shifting aliens the Zygons, who first appeared in 1975.

The story involves a mystery at London's National Gallery in the present day, as well as "a murderous plot" in Elizabethan England in 1562.

Guest stars include Jemma Redgrave as Kate Stewart and Joanna Page as Elizabeth I.

Image caption The Zygons, not seen since the mid-70s, make a return in the anniversary special

Steven Moffat, Doctor Who's lead writer and executive producer, admitted he was "nervous" about the special episode finally being seen around the world.

"I'm glad we don't do it every time, but it's very exciting to do it once," he told the BBC News website.

DOCTOR WHO - THE BRAND BBC Worldwide, the BBC's commercial arm, has sold Doctor Who to 206 territories worldwide including Russia, South Korea, Brazil and New Zealand.

Doctor Who has an estimated 77 million fans in just the UK, USA and Australia alone.

BBC Worldwide has sold over 500,000 tickets to Doctor Who events since 2010 - including The Doctor Who Experience interactive walk-through and exhibition, a Doctor Who Live arena tour and The Doctor Who Symphonic Spectacular.

8m action figures have been sold since the show returned in 2005, plus more than 1m sonic screwdrivers and 10m DVDs. Source: BBC Worldwide

He added he hoped fans would be "very happy" with the 75-minute special. "It's got a big emotional wallop at the end, I think they'll be cheering."

Moffat, along with Matt Smith and Jenna Colman, attended the official Doctor Who anniversary celebration at London's ExCel on Friday.

The three-day event, which is being attended by 8,000 fans a day, features appearances from Doctor Who stars from all eras of the series.

Smith told fans during a question and answer session that Steven Moffat's anniversary story "added to the mythology" of Doctor Who.

Moffat described the first ever Doctor Who episode, An Unearthly Child, broadcast on 23 November 1963, as "one of the very best episodes of Doctor Who ever made".

Media playback is unsupported on your device Media caption Jenna Coleman said she was humbled by fans at London's anniversary event

"All the ideas come from there," he said. "The music, the name, the Tardis, the police box bigger on the inside... in terms of brand new ideas that's a rollercoaster of 25 minutes."

Among fans attending the event was Taylor, 13, from London, a fan since the show was revived in 2005.

"It's a big point in Doctor Who history," she said. "I hope it lives up to the fans' expectations. They'll be expecting a big climax."

American Richard LeCour said he made a special trip from his home in California because Doctor Who had been "part of my life for 40 years".

He said he was "very excited" to see how the story would handle the introduction of John Hurt's Doctor.

Media playback is unsupported on your device Media caption Former sound engineer Brian Hodgson on creating the sound of the Tardis

Reuben Mount, a Matt Smith fan from Brighton, admitted the episode The Name of the Doctor left him "in tears". He added: "I want to know how John Hurt fits into all of it, I want to know the answers."

His friend, Adam Highway, also from Brighton, predicted that Doctor Who had a long future ahead of it.

"It'll go on as long as it keeps that balance of appealing to people who don't know the history, but respects the history for those who give a damn about it. It think Steven Moffat's got it spot on."

The anniversary story is Smith's penultimate outing, before he regenerates at Christmas into a new Doctor, played by Peter Capaldi.