Gormley says it aims to paint a picture of modern Britain -- and provide a contrast with the monuments to historic figures like Admiral Nelson and King George IV which surround it.
"It's not about the past, it's not about the dead, it's not about war, it's about now and about later," Gormley told reporters.
"I hope that we're going to learn... the things we find funny, the things we find scary, the things we fear and the things we love and that's the idea.
"It's a composite portrait of the UK now in all its wonderful, multicultural difference."
Those taking part had varied plans for their hour in the limelight.
Art graduate Suren Seneviratne, 22, was dressed as a panda and carrying a sign featuring his mobile phone number so passers-by could call him, while housewife Rachel Wardell, 35, filled balloons from a helium cylinder.
Nurse Jason Clark, 41, from Brighton, southern England, was one of the first to take part.
"Normally the plinths are reserved for generals, kings, people who have done heroic deeds -- this is a chance for ordinary people to get up there," he said.
"I'm going to get up there and not actually do anything... I'm going to take in the ambiance of the square and take some photos -- and if the worst comes to the worst, I'll sit down and read a book".
Afterwards, he described the feeling of being on the seven-metre high plinth as "invigorating".
The launch of the work was briefly disrupted by a protestor who climbed on to the plinth carrying a banner saying "Save the children. Ban tobacco and actors smoking". Gormley jokingly hailed the protestor as "a warm-up act".
The installation is taking place on the fourth plinth at Trafalgar Square, which was empty for some years but has recently hosted a succession of modern art works by the likes of Marc Quinn and Thomas Schutte.
---------------------------------------------------------------------------------------------------------------