By Lloyd Jones in Sydney

THE Sydney Opera House is being celebrated as “a symbol of modern Australia” as the world-renowned building marks 50 years since the Queen officially opened it in 1973.

A flashy audio-visual display lighting up its famous sails is set for Friday night ahead of a weekend of celebrations as the icon opens its doors to the public.

King Charles III has sent a message of congratulations five decades to the day since his mother, Queen Elizabeth II, officially opened it on October 20, 1973.

“Since its opening in 1973 by my beloved mother, the Sydney Opera House has stood as a continuing legacy for Australians, profoundly influencing contemporary arts and culture, both locally and internationally, and having global recognition as a symbol of modern Australia,” the King’s letter said.

A weekend of celebrations is planned to mark 50 years since the opening of the Sydney Opera House.

“I extend my heartfelt congratulations to you all for a most memorable and successful celebration.”

The world heritage-listed Opera House was designed by Danish architect Jorn Utzon.

He quit the project in 1966 over a dispute with the NSW government and vowed never to return to Australia. He died in 2008.

His eldest children, Jan and Lin Utzon, are in Australia for the anniversary.

The audio-visual laser show on Friday night is just one part of a festive weekend of events that are free to the public.

Opera House chief executive Louise Herron said celebratory parties for the building’s 400 staff and stakeholders were being held on Friday.

On the weekend the building will open to the public for free, with 37,000 booked visitors expected.

“Since the Opera House opened 50 years ago today, it really has changed the nation,” Ms Herron told ABC TV.

“It is the symbol of modern Australia. It’s the most recognised thing in Australia, internationally, apart from the kangaroo and the koala, and the most loved.”

Ms Herron said people saw the Opera House as a place of “boldness and creativity… reflecting the Australian spirit of us being able to do things against the odds”.

The Opera House has hosted famous musical acts for decades and provided a stunning backdrop for visits by monarchs, world leaders, popes, pop stars and protesters.

A newly freed Nelson Mandela in October 1990 gave a speech on the Opera House steps to tens of thousands of people, speaking of forgiveness and thanking Australia for opposing apartheid.

In March 2003, two activists protesting against the war in Iraq famously scaled the building and painted the words “No War” in big red letters at the top of one of the sails.

In 1996, the building provided the backdrop to a goodbye concert by beloved Australian band Crowded House, with up to 150,000 people cramming the forecourt.

The Opera House sails also feature prominently in Sydney’s Vivid festival of light, music and ideas each year and are regularly lit up to mark special events or issues.

Most recently, they controversially showed the colours of the Israeli flag in solidarity with the country’s people following deadly attacks by Hamas.