Wet weather did not stop fans turning out for the 20th anniversary screening of Twin Town on Thursday evening.
Up to 5,000 fans of the Swansea-set cult movie, starring Rhys Ifans, were expected in the city's Singleton Park.
"There'll be a few tears," Allen said ahead of the screening.
The 1997 film centred around tearaway "twins" Jeremy and Julian Lewis, played by Ifans and his real-life brother Llyr Evans.
The plot involved local businessman, rugby enthusiast and part-time drug dealer Bryn Cartwright and corrupt policemen Terry, played by Scott, and Greyo, played by Thomas.
Allen's mother's part was in a group of well-heeled ladies whose request for a taxi to Cwmdonkin Bowls Club is met with a foul-mouthed response.
"Imagine asking your mum to be sworn at!" said Allen. "But she loved being in the film - and we loved having her in.
"So at the screening, it'll be emotional as I'll see my mum again and my uncle Roy, who was a customer at the massage parlour where the Lewis twins' sister worked."
Ceredig, who died in 2011, played Fatty Lewis, the twins' father.
"We'll miss the great Huw Ceredig," said Allen.
"I remember him coming to rehearsals and saying I don't have to say the F-word do I? He couldn't believe it."
Twin Town was Allen's debut feature film but Ifans, who went on to star in Notting Hill and The Amazing Spider-Man, was unable to make the screening.
"He's gutted he can't make it," said Allen, "Rhys has sent a video clip from Berlin where he is shooting a movie. He's made a lovely speech.
"But to think about 5,000 people will come out and watch your film outside, 20 years on and when it might rain, is truly humbling."
The screening included a hot dog van, in honour of an infamous scene in the film where the director's brother, Keith Allen, playing a farmer called Emrys, unwittingly buys a hotdog laced with magic mushrooms from the twins.
But Allen is still searching for the iconic AC Cobra and the two-tone BMW 525, cars in which the Lewis twins were seen joyriding, for the event.
"I had never had imagined Twin Town would have been such a cult film," said Allen.
"It has grown over the years. It's a favourite among the armed forces and rock bands. And outside the UK, it's biggest market is Iran. Where has that come from?"
When Twin Town premiered, the only film to contain more uses of the F-word was Martin Scorsese's Casino and it has been estimated that there are 3.21 uses of the swear word in the film every minute.
"People have asked do I regret the amount of bad language but I don't because it is part of the language of the film," Allen said.
"And that's how some people speak. Every 'F' and blind took a lot of time and honing and polishing, the bad language wasn't thrown in."