"It is without a doubt one of the great moments in Nobel history," the current head of the Norwegian Nobel Committee, Thorbjoern Jagland, told AFP.

"For these 21 years, Aung San Suu Kyi has shown that it was not only justified to award her the prize but she has also shown herself to be a moral leader for the whole world. Even though she spent most of this time in isolation, her voice became increasingly heard."

On October 14, 1991, the Nobel Committee announced it had awarded the Peace Prize to Suu Kyi "for her non-violent struggle for democracy and human rights," propelling the petite democracy champion onto the global stage.

Suu Kyi was under house arrest at the time, after the military junta refused to acknowledge her opposition National League for Democracy's crushing election victory the previous year.

"The regime was not opposed to her travelling abroad (to pick up her prize) but she risked not being allowed to return to her country," said Nobel Committee secretary Geir Lundestad.

He said he believed then, and does now, that her fear was justified.

Before Suu Kyi, only a handful of Nobel laureates had been prevented from travelling to Oslo to collect their Peace Prizes: German journalist and pacifist Carl von Ossietzky, Russian dissident Andrei Sakharov and Polish Solidarity opposition leader Lech Walesa.

In Suu Kyi's case, it was her British husband, Michael Aris, and the couple's two sons Alexander and Kim who accepted the prize on her behalf at the formal ceremony in Oslo on December 10, 1991.

Accepting the award in her stead, her elder son Alexander said: "I know that if she were free today my mother would, in thanking you, also ask you to pray that the oppressors and the oppressed should throw down their weapons and join together to build a nation founded on humanity in the spirit of peace."

His moving words touched many of the guests in Oslo City Hall.

"Listening to Alex, Queen Sonja and (then) prime minister Gro Harlem Brundtland, both of them mothers, were crying. As were many others," Lundestad remembered.

Alarmed by her fate, the international community upped its efforts to secure her release, with calls by international leaders, threats of economic boycotts, and awarding her various prizes and distinctions.

But to no avail.

"She would have been in much more danger if she had not won the Nobel," Jagland said.

Amid a wave of political reforms in Myanmar, Suu Kyi was finally freed last year after spending a total of 15 years on-and-off under house arrest.

Now a member of parliament, she will be able to give the traditional Nobel laureate lecture on Saturday, three days before her 67th birthday.

"It's a lesson in optimism," said Lundestad.

"This just goes to show that in the long run, you can't govern against the will of the people."

After Suu Kyi, the only living laureate yet to pick up his prize is jailed Chinese dissident Liu Xiaobo, who won the prestigious honour in 2010.

"I hope it won't take 21 years for him to collect his prize," Jagland said.

"But the case of Aung San Suu Kyi is an important signal that shows that sooner or later, supporters of democracy end up winning."