The Sydney Harbour Bridge is truly an icon, and has come to represent Sydney in much the same way that the Eiffel Tower is a symbol of Paris.

The idea for a harbour crossing in Sydney initially focussed on a tunnel, and then a cantilever bridge design, before settling on an arch design when tenders were called following World War 1.  The popular belief is that the design was based on that of the Tyne Bridge in Newcastle, but the Tyne Bridge was in fact designed later (although completed earlier).  The 'look' of Sydney's Bridge was actually taken from the Hell Gate Bridge in New York.

Construction of the Bridge by the NSW State Government was achieved with substantial borrowing from Britain.  In response to the Depression, the controversial NSW Premier Jack Lang proposed amongst other measures to suspend loan payments, provoking a crisis in relations with Australia's national Government as well as with the UK.  Lang's official opening of the Bridge on 19 March 1932 was upstaged by a man on horseback (a member of the fascist 'New Guard') who slashed the ribbon with a sword.

Wikipedia presents a reasonable history of the Bridge, and there are some photos of construction work here.

Moving forward to today, or at least yesterday, the NSW State Government staged a huge 75th anniversary celebration for the Bridge, which included closing the Bridge to vehicles from 4am to 11pm to allow a mass walk from north to south across the Bridge.  The anniversary date must have been a very helpful coincidence for a Government facing an election next Saturday, as it allowed them to stir up pride and sentiment about this much-loved icon.

Our family drove to Sydney for the weekend, partly to walk across the bridge.

Here you can see walkers on the northern approaches to the Bridge.  Note how everyone is wearing fluorescent yellow/green caps, handed out for free at the start of the walk.  I suspect they were paid for by the Government facing re-election!  There are some traces of red smoke to the right of the Bridge, left by parachutists with smoke flares who had just dropped from sight:

This photo was taken from the road deck of the bridge, looking up into the arch:

This one looks back from a slight rise on the southern (CBD) approaches, showing the sea of fluorescent caps.  The Bridge is very wide, carrying 8 vehicle lanes, two train lines, and a pedestrian/cycle lane.  The tide of pedestrians continued like this from 10am to 8pm:

And to finish the photographs, here is a view from the Harbour to the east of the Bridge.  It was a heavily overcast day, thankfully preventing walkers from getting sunburnt.  Note that the arch doesn't actually touch the pylons at the end - this is something you don't notice except when viewing the Bridge exactly side-on.  The pylons were apparently built for aesthetic rather than structural reasons: