Celebrations are under way in Sydney to mark the 75th anniversary of the opening of the iconic Harbour Bridge.
The structure, which took nine years to build, was declared open for traffic in March 1932 and remains to this day the world's largest steel arch bridge.
More than 200,000 people are taking the rare opportunity to walk across a structure normally filled with traffic.
The bridge, known affectionately as the Coathanger, has 6m rivets in its 52,000 tonnes of steel.
The BBC's Nick Bryant in Sydney says that never before has the bridge been closed to vehicles for so long - a move intended to give as many Sydneysiders as possible the chance to cross.
A blast of a didgeridoo marked the start of the celebrations, which include fly-pasts, a regatta and Aboriginal sunset ceremonies.
However there will be no fireworks, our correspondent says, as the organisers do not want to distract from the magnificence of the bridge.
The celebrations also had a moment of commemoration - a plaque being unveiled to honour the 16 workers who died during its construction.
One walker on Sunday was Bruce Boddington, who at four was the youngest person to walk the bridge on its opening day.
He told Reuters news agency: "It's wonderful, seeing the crowd. They've all got happy looks on their faces."
The bridge was finished in the Great Depression and was sometimes called the iron lung because it helped breathe life into a city and country experiencing terrible economic hardship.
The design of the bridge still arouses debate - both Australian engineer John Bradfield, who oversaw the project, and British consultant engineer, Ralph Freeman, are cited. Both are on the opening plaque.
I was almost 10 years of age on the day that the bridge was opened and about 7.30 pm that evening I walked with my father across the bridge as did it seem to me at the time so did most of the inhabitants of Sydney. Earlier that day [about 10.30AM] it had been opened incorrectly by an Australian army officer on horseback [Captain Francis de Groot] swinging a rather lengthy sword with which he slashed open the official red extended tape, and much to the annoyance of our British Royal governor [Philip]. The officer was pulled off his horse and was arrested by Police officers who happened to be in attendance. Accordingly the red slashed red tape was knotted at the cutting point and the British New South Wales Governor proceeded to cut the damaged red tape and officially opened the bridge.
Edward Dalton, Sydney-Australia
I walked across the bridge today with my 93 year-old grandfather, Eric Brown OAM, who walked across the bridge when it was first opened. I'm hoping that I'll be able to walk across the bridge on its 150th anniversary unassisted like he did today.
Anthony, Sydney, Australia
We walked the bridge this afternoon as the F1-11s flew over, it was awesome! Only disappointment was that they had run out of souvenir hats! We're going to have to come back in 25 years time for the centenary celebrations to make sure we get one then! Let's hope we beat them at cricket in the world cup!
Stuart Atkinson, Ipswich, UK
My late Father was Master of one of the first British merchant ships "City of Khios" to pass beneath the bridge after its opening
Will Reay, Bangor Co.Down Northern Ireland
It was a pretty interesting experience - although you realise how relatively small the distance really is when you walk it (it took about an hour, at a very leisurely pace)... There was a lot of ridiculous pomp, and they were playing music from enormous speakers; the Bee Gees, the Seekers and Midnight Oil... Quite a day for Sydney!
Emma, Glebe, Australia
I kissed Bine underneath the Sydney Harbour Bridge last February. It was one of the happiest moments of my life. Thank you Sydney!
Elly Campbell, Sydney
I've climbed up, cycled across, got the train, sailed under and driven over the mighty bridge, but never have i got to walk on its road. Lucky Sydney people, maybe ill be around for the 100 year anniversary!
Niall O'Callaghan, Dublin, Ireland
We took our 6 week old son Lucas over the bridge in the pram. It's an impressive piece of architecture, it wasn't too hot & he slept all the way too. A great day.
Tim & Fiona, Sydney, Australia
Climbed the bridge in summer of 2004 as a present from parents for studying hard for school GCSE exams, the greatest experience of my life, also saw a guy propose 2 his girlfriend at the top, and she said yes! its amazing! Sydney harbour bridge is beautiful and so is AUSTRALIA!!!
Rehan Ansari, Woking, Surrey
I came to Sydney in 1980 from Cardiff and remember the euphoric feeling that I got when I drove across the Harbour Bridge for the first time. The Bridge is enormous and radiates a special iconic vibe. I really felt on that day that I had truly arrived on the other side of the world ... then as I drove off the other side of the bridge I caught sight of the Opera House ... that fabulous view only increased the intensity of the moment. I had always wanted to see the Opera House and there it was! Today I walked across the Bridge on roads that I have driven over to go to work so many times. The smiles and friendliness of the thousands walking with me (and my Aussie family) made me realise that I made the right decision to come here all of those years ago. I love this Harbour City.
vic stephens, sydney, australia
It was my first memory of Sydney, seeing it as the plane crossed the CBD as it landed. Now I see it everyday from the street my office is on but it doesn't lose its appeal. Walking across it provided a different perspective on it but it is best viewed from the harbour on the water.
Mark Stewart, Sydney, Australia
We walked the bridge just as the 747 came over - fantastic!
Kelly and Kevin, Chelmsford, Essex
My mother-in-law walked across opening day in '32 as a young girl.....we were downtown today staying in a nearby hotel and saw all the people flooding into the area.....the bridge was closed for around 12hrs which is un-heard of....there was, somewhat, an eerie quiet downtown SYD.....brought about, of course, by closing ----- that rather large steel single-span structure!!
Michael Murray, Ettalong Beach, Australia..
The Sydney bridge was a kit made in North East England and only assembled in Australia. It is Tyneside technology. You wouldn't give you kid brother the credit for building a ship when he only assembled an airfix model would you? Chris Rea's song "From Sydney harbour to the Frisco bay" summarises the work done by the lads in the North East of England.
Davy, Ronda, Spain
Actually Davy, Ronda, Spain. The Sydney Harbour Bridge is the work of TEESSIDE technology not Tyneside, it's the work of Dorman Long based in Middlesbrough and the bridge clearly notes this FACT on a plague featured on the bridge itself.
R. M. Sutcliffe, Middlesbrough, Middlesbrough
My father was named after the city because the bridge was completed in the year he was born - 1931. His grandparents were living near the city at the time. Sydney died in 2006, aged 75.
Stuart Woodward, Colchester, UK
In January 2000, my son Michael was married on a Sydney Harbour Ferry, the Charlotte, directly underneath the Sydney Harbour Bridge. I believe this was the first wedding of the New Century under the bridge. Obviously a happy time and full of unique memories. Best wishes Beryle Ash (Mrs)
Beryle ash, Abu Dhabi, UAE
This is my first experience on the Bridge. I am from India and is here as a student. Great experience to walk on this historical Bridge.
Venugopal Kannan Meenakshisundaram, Sydney