The British Film Institute has apologised after staff forcibly removed a woman with Asperger syndrome from the cinema in what onlookers described as a “disgusting” sign of “naked intolerance”.

Tamsin Parker, 25, an artist and animator, was watching a screening at the BFI’s cinema on London’s South Bank of her favourite film – the spaghetti western The Good, the Bad and the Ugly, directed by Sergio Leone – with two friends. She was asked to leave for laughing too loudly.

Parker’s mother, Lydia Parker, said some members of the audience applauded as she was removed: “[My daughter] said ‘I am autistic’ and a man said: ‘You’re retarded.’ Another man, who called her a bitch [for laughing], was thrown out, but only after she was.”

Her mother added: “She was completely humiliated and it ruined her birthday.”

The BFI released a statement saying it was sorry and had got it wrong in a “challenging and complex situation”.

“We are taking this situation extremely seriously and this morning we have been investigating further ... We can and must do better in accommodating all the needs of our customers and we will be addressing what additional provisions and staff training we can put in place,” the institute said.

Parker’s mother, who is speaking to a lawyer about what happened, said: “It would be nice to arrange a screening for Tamsin and her friends. The manager did not seem very apologetic at the time.”

Parker was eager to see the BFI screening and has watched the film eight times, according to her mother, who went to collect her daughter after she was thrown out. “She was in floods of tears ... she was really scared,” her mother said.

Lydia Parker, a theatre director, said it had been heartening to see messages of support on Twitter. The story has been shared widely on social media, with people voicing concern.

One cinemagoer, Lloyd Shepherd, expressed his disgust after the incident. “She’d been laughing very loudly, but at moments which were supposed to be funny. Some people complained. She was dragged out shouting: ‘I’m sorry, I have Asperger’s.’ She was incredibly upset,” he said.

He said some people applauded her removal while others were upset, with a large number leaving.

“I am shaking with anger. That poor, poor woman. Just a little bit of empathy and everything would have been fine. Such naked intolerance. In the middle of London. Disgusting,” Shepherd said.

Other witnesses tweeted:

Parker’s sister, Sabrina Parker, tweeted that she had taken her sibling home to watch the rest of the movie. “Thank you for sharing this ... we’re horrified that they would treat her so badly,” she wrote. “Obviously she’s still very upset. It’s her favourite movie and it was her 25th birthday celebration.”

The National Autistic Society said many autistic people felt venues were not autism-friendly enough, which is why it runs the autism friendly award to encourage businesses to be more accessible. The charity added that it works with cinemas and theatres to hold autism-friendly screenings.

Jane Harris, the NAS’s director of external affairs, said the incident was shocking and a colleague was at the screening, “along with others who were equally distressed by what they had witnessed”.

“It’s great to hear that so many audience members were sympathetic, but this incident shows just how far we have to go for autistic people to get the understanding they deserve,” she added.

Harris said the NAS would be reaching out to the BFI to discuss ways to improve its staff members’ understanding of autism.