The Kent-born Australian actress looked flawless in a white Versace dress for her big night on the red carpet.
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Tasked with the unenviable task of portraying one of the world's most beloved icons, Naomi looked suitably regal in her classic gown, teamed with drop earrings and black peep toe heels. Onlookers noted there was a strong sense and look of Diana about the 44-year-old star as she signed autographs for delighted fans.
Speaking on the red carpet, Naomi revealed she had initially turned down the part twice, but felt compelled to take on the role because she found the late Princess so fascinating.
"Any woman I take on, I want them to be all things," she said. "I'm fascinated by women who are complex, full of contradictions, who are fun, intelligent but frail at the same time.
"Someone who is going through something, some kind of transformation, and this story is definitely that."
Diana focuses on the last few years of the royal's life and chronicles her secret love affair with Pakistani heart surgeon Hasnat Khan (Naveen Andrews). Naomi did a great deal of research before taking on the role, and said she found the Princess to be a very funny woman.
"It seemed to me she had a great spirit and a great sense of humour and a mischievous way," she told the Metro.
Given its focus, the film has courted controversy in the lead up to the premiere with Naomi admitting she was "afraid to play the most famous woman of our time because of all the comparisons."
Speaking on the red carpet, she said that if Diana's sons Prince William and Prince Harry did see the film, she hoped they would feel it was made with the utmost respect.
"I had my concerns going into it, which is why I hesitated," she told the Mail. "But the film is done. Whether or not they (William and Harry) see it, I have no idea. But if they do, I hope they feel that we have been respectful, and upheld her memory in the best possible way."
Also on the red carpet were Naveen, Douglas Hodge, who stars as Paul Burrell, and director Oliver Hirschbiegel. Diana is released in cinemas on 20 September.