Kim Moon-soo won the People Power Party's nomination after shooting to public attention as the only cabinet member who refused to bow in apology for failing to prevent Yoon's attempted suspension of civilian rule and for opposing his impeachment.

The former student labor activist-turned-conservative secured the nomination with 56.5 percent of the vote, a result based on a mix of party member ballots and public opinion polls -- defeating former party chief Han Dong-hoon.

"We have gathered here today, pushing through frustration and anger. Despite the cries of countless citizens, our president [Yoon] was impeached," the 73-year-old said in his acceptance speech.

"Even if my body is shattered to pieces, I will win -- no matter what."

The primary was a showdown between Kim, who opposed Yoon's impeachment, and Han, a former Yoon ally who backed the president's impeachment despite facing a backlash within his own party.

Yoon's martial law decree lasted just around six hours before defiant lawmakers scaled parliament's fences to convene and vote it down.

The ex-president's move has been widely condemned as unconstitutional and led to his impeachment by the National Assembly and subsequent removal from office by the Constitutional Court.

But on Saturday, Kim criticized the opposition-controlled parliament, accusing it of unfairly "impeaching a president elected by the people".

Read also: South Korea's Yoon removed from office over martial law, election looms

After enrolling at South Korea's prestigious Seoul National University, Kim spent around two decades as a labor and pro-democracy activist fighting military authoritarian regimes -- including serving a jail term -- but later said he shifted his views following the collapse of the Soviet Union in 1991.

"I, Kim Moon-soo, have always lived with passion, starting from the humblest beginnings. As a child, my deepest wish was just to have a warm meal," he said during his acceptance speech.

"I never turned my back on the vulnerable or those at the bottom of society."

The latest Gallup poll, released on April 25, shows main opposition Democratic Party frontrunner Lee Jae-myung, currently facing multiple criminal trials, leading the race with 38 percent support, while all other candidates, including Kim, trail in the single digits.

Former acting president Han Duck-soo -- a 75-year-old career bureaucrat and ex-prime minister under Yoon -- resigned on Thursday and the next day officially announced his presidential bid.

He is expected to team up with the People Power Party to launch a unified conservative campaign against Lee and experts say he could eventually emerge as the PPP candidate. 

South Korea's top court on Thursday ordered a retrial for Lee over alleged election law violations, with both parties being required to register their official candidates by May 11.

PPP candidate Kim on Saturday called Lee -- who won his party's  nomination with an overwhelming 89.7 percent of the vote -- a "dictator".

"I will form strong alliances with any group necessary to stop Lee Jae-myung and his Democratic Party faction from taking power," he said.