The French firefighter who treated Princess Diana as she lay fatally injured in a Paris tunnel has recalled her final moments as mourners flock to Kensington Palace to mark the 20th anniversary of her death.

One of the most famous women in the world, Diana was killed aged 36 on August 31, 1997, when her limousine crashed in a Paris tunnel as it sped away from paparazzi giving chase on motorbikes.

Sergeant Xavier Gourmelon, who led the response team to the tunnel crash, told The Sun newspaper in a story published Thursday night (AEST) he was certain she would survive the crash.

Sergeant Gourmelon said the Princess of Wales’ last words were: “My God, what’s happened?”

“I could see she had a slight injury to her right shoulder but, other than that, there was nothing significant. There was no blood on her at all,” he said.

Sergeant Gourmelon said the Princess was conscious and had her eyes open when he pulled her out of the wrecked Mercedes.

“To be honest, I thought she would live. As far as I knew when she was in the ambulance she was alive and I expected her to live.

“But I found out later she had died in hospital. It was very upsetting.”

The man who treated Diana 20 years ago thought she would survive the crash. Photo: Getty

Sergeant Gourmelon also recalled how Diana’s bodyguard, Trevor Rees-Jones, kept trying to reach the Princess from the front seat of the car, even though he was also injured.

“He kept asking for the Princess, saying, ‘Where is she? Where is she?'” he said.

Sergeant Gourmelon said he has not spoken publicly about that crash and its aftermath for 20 years because he continued serving in the fire service until only recently, and was not allowed to speak to news outlets about his experience.

Princess Diana mourners gathered at the gates of Kensington Palace from before dawn local time to celebrate the life of a woman who’s death transformed Britain and its royal family.

Diana’s death prompted the biggest outpouring of grief seen in Britain in recent times and hurt the monarchy, which was accused of reacting coldly.

The events that followed, when Queen Elizabeth promised to learn from Diana’s life, are regarded as a turning point in the modernisation of the country and how the royal family relates to the public.

On the anniversary of her passing, fans gathered in the dark to lay candles and remember a woman who married Prince Charles, the heir to the throne, in a glittering 1981 wedding ceremony before their bitter divorce in 1996.

Diana’s death is credited with forever changing Britain and its relationship with the royal family. Photo: Getty

“She brought a breath of fresh air to the royal family,” said Caryll Foster, 57, who got to the palace for 3:00 am to mark the time when the news of Diana’s death broke.

“The royal family can be a bit cold, and she was warm hearted and kind. She was very special and we want to keep her memory alive.”

Mourners also paid tribute by gathering at the site of the car crash in Paris.

Paris Mayor Anne Hidalgo arrived at the Alma Tunnel on Thursday to lay a large bouquet of flowers near the spot of the fatal incident, according to the Associated Press.

Fans also left flowers, notes and photos remembering the Princess of Wales.

Music icon Elton John, who memorably performed his moving song Candle In The Wind at Diana’s funeral, offered a tribute to the late princess, posting an image on social media.

John posted a photo of the pair laughing together and with his hand on Diana’s shoulder, and wrote: “20 years ago today, the world lost an angel. #RIP.”

The 20th anniversary of Princess Diana’s death has prompted a renewed fascination with the “People’s Princess”, as she was dubbed by then-Prime Minister Tony Blair, with her two sons Prince William and Harry leading the tributes.

The front pages of newspapers showed the two royals examining flowers and notes left for their mother, next to the same image taken 20 years ago when, aged 15 and 12, they met mourners and read some of the thousands of messages left at the gates.

In documentaries broadcast in recent weeks, the two men have spoken about the trauma they suffered from their mother’s death and their sense of confusion and bewilderment at the country’s grief.

Diana’s remarkable life in pictures. Watch below:

– With AAP