For Philip Larkin the annus mirabilis was 1963 when "sexual intercourse began". But Rome was a couple of jumps ahead, and this month Italy is celebrating the start of the "Dolce Vita" years which began, all agree, 50 years ago this month, in November 1958, with an impromptu striptease in a Roman trattoria.

La Dolce Vita, "The Sweet Life", was the title of one of Federico Fellini's most seductive films and naturally grew to encompass the liberated era that inspired it.

Suddenly everything became possible, to the shock of many at the time. In 1958 Olghina di Robilant was a penniless young Venetian countess struggling to make a living in the Italian capital, and that November her rich friend Peter Howard Vanderbilt agreed to cheer her up by bankrolling a birthday party for Olga – her 24th.

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Now 74, she remembers Rome as safe, cheap, innocent and puritanical. "There were no crowds, no pushing, no corruption, no unsavoury ambition. The paparazzi, for example, took photos with the agreement of the stars." She threw her party in Rugantino, a trattoria in Trastevere, and one of the guests was the commissar of the local police station.

"The party was going very nicely when Peter Howard said to me, 'Come and see who's sitting at the bar.' It was this woman caked with make-up like a clown and wearing very little clothing – an Armenian dancer called Aïché Nana. I said, 'I don't remember inviting you' – and she ran to the dance floor and started dancing by herself, a thing nobody did. Then Anita Ekberg, the Swedish film star, arrived, without Fellini, though I had invited him, and Anita on the dance floor started pulling down her suspenders. And then the Armenian woman started stripping, and suddenly the flash bulbs were popping and the police commissar said, 'Stop her!' and I said 'You're the policeman, you stop her!' So he did, and they threw her out. The commissar made the photographers hand over their film. And then the party resumed and went on until six in the morning."

But one of the photographers hung on to his shots of the striptease, and next morning Olghina woke up to find the papers screaming about the "Roman orgy at Rugantino's". "It was quite untrue," she said, "but it took on a life of its own and went on like a tsunami."

The photographer was called Tazio Secchiaroli. Thanks to Cinecittà, the film production studios on the east side of the city, Rome had become a popular location for Hollywood films, and the foreign stars and writers began hanging out in the bars of Via Veneto. "My father was a news photographer," says Davide Secchiaroli, "but he and a few friends decided to go to Via Veneto and photograph the stars there." His work documented the city's unique mix of aristocratic tradition, religious superstition and celebrity glitz – and then he ran into Fellini and it was a meeting that changed everything. Fellini put him into his film as the jobbing snapper Paparazzo – the coinage that named an entire tribe – and his random daily patchwork of jobs became the film's narrative frame.

"The reality was first photographed by my father," says Mr Secchiaroli, whose father died in 1998, "then transformed by Fellini's great imagination."

Aïché Nana's striptease is immortalised in the film, but perhaps the most famous scene is that in which Marcello Mastroianni and Anita Ekberg wade at night into the waters of the Trevi Fountain. This, too, was borrowed from life. "I was the first to dive into the fountain," boasts Olghina di Robilant. "It was November and freezing. I didn't have two pennies to rub together, and one of my friends said, 'I'll give you 10,000 lire if you jump in.' So I just dived in, grazing my nose on the bottom, and it inspired the scene in the film."

Today all the old-timers agree that Rome has gone to the dogs. The crowds and permanent police guard make diving into Trevi Fountain an unattractive proposition. Rugantino's trattoria is now a McDonald's, while Via Veneto, according to Enrico Lucherini, agent to the stars, "is terrible now, full of shoe shops and tourist restaurants charging €15 for a coffee".

The film caused a huge scandal when it came out, and narrowly avoided being banned. At the premiere, one outraged signora spat in the director's face. "We were furious with him," says Olghina, "because it wasn't a decadent city. Fellini, who comes from Rimini, based the film on gossip. He wasn't yet part of the city's life – he was like a maid looking through the keyhole." Yet no one denies that Fellini crystallised an amazing vision of Rome.

At least one person was in Fellini's debt for the rest of his life: Tazio Secchiaroli, the original paparazzo, became the director's privileged stills photographer. Far from ambushing stars, he became personal photographer to Sophia Loren for more than 20 years.