Early voters can cast ballots at about 3,510 polling stations across the country before the election next Tuesday, the National Election Commission said in a statement.
It's the ROK's first presidential election with early voting after introducing it for parliamentary and mayoral elections in recent years, the statement said.
Pre-election surveys show liberal candidate Moon Jae-in comfortably leading his two main rivals - a centrist and a conservative.
The winner will be sworn in as the new president immediately, forgoing the usual two-month transition. Park's impeachment and removal from office changed the ROK's election schedule, so the new president will serve one full five-year term.
Park is currently jailed at a detention center near Seoul awaiting her trial on allegations that she extorted money from businesses, took kickbacks from some of those companies and committed other wrongdoing, all in collaboration with a longtime confidante. The trial is to formally start later this month.
A commission-run website showed about 4 million people had voted by midafternoon. The ROK has 42,479,710 eligible voters, according to the election commission.
Higher turnout
Many observers believe disquiet over the recently installed US missile defense system, known as THAAD, may have a major role to play in the election, especially among younger voters.
A higher turnout, especially among those in their 20s to 40s who have historically been indifferent to politics, is seen boosting Moon, who wrote in a book published in January that the ROK should learn to say "no to America".
Unless there's a major upset, Moon, who advocates a moderate approach on the Democratic People's Republic of Korea and criticizes conservatives and their hardline policies, will become the next president.
A Gallup Korea poll published on Wednesday shows him with 38 percent support in the field of 13 candidates, with centrist Ahn Cheol-soo his nearest challenger at 20 percent.
"Sometimes I feel our government is bending over backwards trying not to upset the alliance with the United States," said Jason Lim, a 36-year-old South Korean engineer living in Washington.
"I want our president to be someone who isn't afraid to say what's best for Korea."
