The attempt to impose martial law by former president Yoon Suk Yeol threatened an irreparable setback to the country, but the people rose up and stopped the military with their bare hands, Lee said in an address marking the first anniversary of the shock announcement.
"The recklessness of those who tried to destroy constitutional order and even plan a war all for their personal ambition must be brought to justice," he said.
"The December 3 coup d'etat was not just a crisis for democracy in one country. If democracy in South Korea collapsed, it would have meant a setback... for world democracy."
Yoon's martial law declaration plunged a country that had been viewed as one of the success stories of democratic resilience into months of political turmoil, just as US President Donald Trump's decision to slap tariffs on global trading partners rattled South Korea, an export-heavy economy.
The conservative leader was later ousted and Lee, a political rival who lost to Yoon in a 2022 presidential poll, won a snap election in June with a mandate to steer the country out of the shock of martial law, as those who were accused of being involved were arrested and tried for subversion.
Since coming to office, Lee has managed to strike a US tariff deal after two summits with Trump, but there remain deep fissures in society and concerns over whether the conservative side feels it is being persecuted.
Lee said the work of reforming the country following the martial law crisis was bound to be painful and time-consuming.
"But just as treating cancer by removing the cancer cells that have taken root deep inside the body, it cannot be completed that easily," he said.
Yoon justified his short-lived martial law by accusing "anti-state forces", including Lee's Democratic Party, of paralysing government and ruining democracy.
He said he had no choice but to impose martial law to restore order in a surprise late-night announcement on December 3 last year.
On trial for insurrection and facing life imprisonment or even potentially the death penalty, Yoon has denied ordering the arrest of opposition politicians and political enemies and argued the martial law declaration did not harm the country.
A number of former cabinet members, senior military officers and lawmakers are also on trial or under criminal probe. Yoon's wife, Kim Keon Hee, faces a separate trial on charges of corruption and bribery.
Lee said he will propose designating December 3 as a national holiday to celebrate the role of the people in quelling the martial law bid and added that he believed they deserved to be considered for the Nobel Peace Prize.
Lee plans to take part in a citizens' march later on Wednesday to mark how the country defied the attempt to bring in military rule.
