A South Korean court has sentenced the former president Yoon Suk Yeol to life imprisonment with labour over his failed martial law declaration in December 2024, finding him guilty of leading an insurrection and making him the first elected head of state in the country’s democratic era to receive the maximum custodial sentence.

The Seoul central district court found that Yoon’s declaration of martial law on 3 December 2024 constituted insurrection, carried out with the intent to disrupt the constitutional order.

Judge Jee Kui-youn said the purpose was “to send troops to the national assembly to blockade the assembly hall and arrest key figures, including the assembly speaker and party leaders, thereby preventing lawmakers from gathering to deliberate or vote”.

In sentencing Yoon on Thursday, the court pointed to his lack of apology throughout the proceedings, his unjustified refusal to attend hearings, and the massive social costs his actions inflicted on South Korean society.

The court said the martial law greatly damaged the political neutrality of the military and police and caused South Korea’s political standing and credibility in the international community to decline, leaving society “politically divided and experiencing extreme confrontation”.

The court opted for life imprisonment over the death penalty, noting that while the crime was grave, Yoon’s planning did not appear meticulous, he had attempted to limit the use of physical force, and most of his plans ultimately failed.

In a historical digression, the judge traced the history of insurrection law and cited the 1649 execution of England’s Charles I, who led troops into parliament, to establish that even heads of state can commit insurrection by attacking the legislature.

The verdict was broadcast live on national television, capturing Yoon briefly smiling on arrival and later displaying no visible reaction as the sentence was delivered.

Under South Korean law, the charge of leading an insurrection carries three possible penalties: death, life imprisonment with labour, or life imprisonment without labour.

Prosecutors had sought the death penalty, arguing that Yoon committed “a grave destruction of constitutional order” by mobilising troops to surround parliament and attempting to arrest political opponents during the six-hour crisis.

The verdict came 14 months after events that marked the most serious threat to South Korea’s democracy in decades.

The charges stem from events on the night of 3 December 2024, when prosecutors said Yoon attempted to use military force to paralyse the legislature, arrest political opponents and seize control of the national election commission. Yoon claimed he was rooting out “anti-state forces” and alleged election fraud without providing evidence.

Within hours of the declaration, 190 lawmakers broke through military and police cordons to pass an emergency resolution lifting martial law. Parliament impeached Yoon within 11 days, and the constitutional court removed him from office four months later.

Outside the courthouse, hundreds of Yoon supporters – waving South Korean and US flags and chanting “Yoon again” – initially cheered when the judge dismissed some prosecution evidence but turned hostile as the ruling progressed. Some supporters shouted “political judge, step down” and hurled profanities at journalists.

When the sentence was announced, some collapsed in tears, crying “the country is finished”. About 500 metres away, progressive groups erupted in cheers and embraced one another, though some expressed disappointment the death penalty had not been imposed.

Yoon faces six additional criminal trials, two of which arise from the martial law crisis, including a treason charge alleging that he ordered drone incursions into North Korean airspace in an attempt to provoke a confrontation that could justify military rule. He has already been sentenced to five years’ imprisonment for obstructing his own arrest.

Thursday’s verdict follows a series of related rulings that formally established the events of 3 December constituted an insurrection.

In January, the former prime minister Han Duck-soo was handed a 23-year prison sentence in a ruling that described the martial law attempt as a “self-coup” by elected power that was more dangerous than traditional uprisings. The sentence far exceeded prosecutors’ 15-year demand, indicating judicial willingness to impose severe penalties.

On 12 February, the former interior minister Lee Sang-min was jailed for seven years for his role in the insurrection, including relaying Yoon’s orders to cut power and water to media outlets.

Legal experts said the rulings created a sentencing environment that made the most severe punishment more likely in Yoon’s case.

The court also sentenced seven co-defendants: Kim Yong-hyun, the former defence minister, to 30 years; Noh Sang-won, a former intelligence commander, to 18 years; Cho Ji-ho, the former police chief, to 12 years; and Kim Bong-sik, the former Seoul police chief, to 10 years. Mok Hyun-tae, a police commander, received three years. Two defendants, Kim Yong-gun and Yoon Seung-young, were acquitted.

Yoon’s legal team released a statement calling the verdict “a predetermined conclusion” and a “show trial”, saying they could not bring themselves to respect the judgment.

They accused the judiciary of “kneeling to incited public opinion and political power” and applying double standards, pointing to President Lee Jae Myung’s suspended trials and opposition politicians acquitted on illegal evidence grounds.

The legal team vowed to fight “to the end”, saying truth would eventually be revealed “in the court of history”. Yoon is expected to appeal.

Life imprisonment carries no fixed release date, with parole theoretically possible after 20 years, on demonstration of good conduct and remorse.

The former president Park Geun-hye was initially sentenced to a combined 32 years in prison for corruption and related offences in 2018. The term was later reduced on appeal and ultimately erased by a presidential pardon in 2021.

In 1996, the military dictators Chun Doo-hwan and Roh Tae-woo received death and 22-and-a-half-year sentences, respectively, for their roles in a 1979 coup and subsequent massacre in Gwangju. The sentences were later reduced on appeal, and both men were eventually pardoned.

Every South Korean president who has served a prison sentence has ultimately been pardoned.