Thousands thronged Vienna's landmark cathedral Wednesday to mourn Formula One legend and Austrian national hero Niki Lauda at a ceremony to be attended by a host of F1 stars including world champion Lewis Hamilton.

The casket arrived at St. Stephen's Cathedral in the city's historic centre with a police motorcade. Pallbearers, followed by Lauda's family, carried the coffin inside as bells rang out and thousands gathered in pouring rain outside.

The three-time F1 world champion, who came back from a horrific crash and founded several airlines, died on May 20 at the age of 70.

"The whole world says today: Bye, Niki!" tabloid Oesterreich said on its front page.

Lauda's widow Birgit along with two of his sons put his helmet on top of the brown casket, which stood next to flower wreaths and a portrait of the Austrian sporting his signature cap hiding scars from the 1976 crash.

Mourners then slowly filed past to bid a final farewell, taking photos and laying flowers, with the queue stretching well out of the cathedral into nearby streets.

- 'An example for Austria' -

A public mass will take place later Wednesday. The funeral itself is private and details for it have been kept under wraps.

Some 500 VIP guests are expected to attend the mass, including Hamilton and other Formula One heavyweights such as German driver Sebastian Vettel and Mercedes chief Toto Wolff.

There will also be hundreds more seats and 3,000 standing places open to the general public.

It is the first time a sports star has received such an honour at the Gothic Roman Catholic cathedral, a distinction previously accorded only to prominent personalities like the archbishop of Vienna, Cardinal Franz Koenig, and the former crown prince of the Austrian-Hungarian empire, Otto von Habsburg.

Among the thousands of mourners, some sporting Ferrari caps, was Roland, a 41-year-old technician from Vienna.

Waiting in line outside the cathedral, he told AFP that Lauda was "an example for Austria".

"Even though he faced very difficult circumstances, he never complained," he said.

Many had travelled from across the country and even further afield abroad to attend the funeral.

Johann, a 70-year-old retiree from Burgenland state who used to work as a Lauda Air technician, described Lauda as a "strict but fair" employer.

Imre Varga, 63, came all the way from Hungary for the ceremony. She said she would watch Lauda on Austrian television, which people in the western part of her country had been "lucky" to receive in Communist times.

"During the Iron Curtain he was always a role model for us... He was unreachable, like a God, to us," she told AFP.

Lauda won the drivers' world championship in 1975 and 1977 with Ferrari and in 1984 with McLaren

Even after retiring as a race driver, Lauda remained a fixture on the circuit. Most recently, as non-executive chairman at Mercedes F1 since 2012, he was instrumental in bringing in Hamilton, sparking a run of success for the team.

The Briton won the Monaco Grand Prix Sunday, which he described as the "hardest race I think I've ever had".

- Favourite songs -

Former F1 drivers Gerhard Berger of Austria and France's Alain Prost are expected to speak, as is Austrian President Alexander Van der Bellen and former California governor and Austrian-born actor Arnold Schwarzenegger.

During the mass, songs, reportedly Lauda's favourites, including "Amazing Grace", "Fast Car" by Tracy Chapman and John Lennon's "Imagine" will be played.

Lauda died at the University Hospital Zurich in Switzerland, nine months after he underwent a lung transplant.

He had struggled with health issues since miraculously surviving the 1976 crash that saw his car engulfed in flames at the Nuerburgring in Germany.

Lauda also underwent kidney transplants.

One of the kidneys was donated by his last wife. Lauda married the former flight attendant in 2008 and they had twins, a boy and a girl, in 2009.

Lauda also has three other sons from previous relationships.

Besides racing, Lauda's second passion was aviation. He founded and then sold several airlines with his latest, Laudamotion, going to Ryanair in 2018.

It is not clear where Lauda will be laid to rest. The family reportedly has turned down an offer by the city of Vienna to receive a "grave of honour" in the city's vast Central Cemetery.