South Koreans are heading to the polls to choose a new president after Park Geun-hye was sacked and indicted for corruption, and against a backdrop of high tensions with the nuclear-armed North.
More than 139,000 voting stations opened on Tuesday morning across the country.
One in four voters cast ballots in early voting last week, and officials think higher participation by younger people could drive turnout to the highest in three decades.
Voters have been galvanised by anger over the sprawling bribery and abuse-of-power scandal that brought down Park, which increased frustration over jobs and slowing growth.
Left-leaning Moon Jae-in, a former human rights lawyer, has held a commanding lead in opinion polls for months, with the final Gallup Korea survey before a week-long pre-election blackout giving him 38 percent support, followed by former tech mogul Ahn Cheol-soo on 20 percent.
Hong Joon-pyo, of Park's Liberty Korea Party, languished in third place in the 13-strong field on 16 percent.
Se-Woong Koo, managing editor of the Korea Expose, said the events that led to Park's downfall had energised the electorate.
"People have seen what they can achieve by taking to the streets and demanding that their voices be heard," he told Al Jazeera. "They want to continue the momentum, they want to cast their votes and we're expecting a rather big turnout, unless the weather somehow interferes with this process."
It was a cold and cloudy election day in Seoul, where voters came out under overcast skies.
Chung Tae-wan, a 72-year-old doctor, cast his vote at a polling station in prosperous Seocho district in southern Seoul. "I voted for Hong, as security [against North Korea] is the most important thing", he told AFP.
Kim Kyung-min, 24, said she cast her ballot in advance last week. "I was so disappointed in Park and the establishment", she told AFP, but refused to say whom she voted for.
North Korea tension
The campaign has focused largely on the economy, with North Korea less prominent, but after a decade of conservative rule a Moon victory could mean a sea change in Seoul's approach towards both Pyongyang and key ally Washington.
The 64-year-old - who is accused by his critics of being soft on the North - has advocated dialogue to defuse tensions and to bring it to negotiations, and is seen to favour more independence in relations with the US, Seoul's security guarantor with 28,500 troops in the country.
Seoul needs to "take the lead on matters in the Korean Peninsula" and South Koreans should not "take the back seat", he said in a recent media interview.
The North has carried out two nuclear tests and a series of missile launches since the start of last year in its quest to develop a missile capable of delivering a nuclear warhead to the US mainland.
Washington has said military action was an option, sending fears of conflict spiralling.
Moon has said that he would also be willing to visit Pyongyang to meet Kim and advocated resumption of some of the inter-Korea projects closed down by his predecessors, including the Kaesong joint industrial zone.
But for many South Korean voters, corruption, slowing growth, unemployment and even air pollution from China top the list of concerns.
South Korea's rapid growth from the 1970s to 1990s pulled a war-ravaged nation out of poverty but slowed as the economy matured, and unemployment among under-30s is now at a record 10 percent.
Frustration over widening inequality in wealth and opportunities fuelled anger over Park's scandal, which exposed the cosy and corrupt ties between regulators and powerful family-oriented conglomerates, known as chaebols, that have endured for decades.
Park is awaiting trial over corruption charges for offering governmental favours to top businessmen - including Samsung heir Lee Jae-Yong - who bribed her secret confidante, Choi Soon-sil.
BJ Kim, an adjuct professor at Hankuk University of Foreign Studies in Seoul, told Al Jazeera that integrity will be an issue after Park's departure, although the five major candidates have not had major issues of integrity brought up against them.
"Integrity will be a key priotity in the minds of the people but, in terms of making specific choices on whom they are going to vote for, I don't think it's going to be on top of their list."
Moon, Ahn and other candidates have both promised to reform the chaebols, which dominate the economy and have long been criticised for operating with little scrutiny from investors or regulators.
China is South Korea's top trading partner - and also a major source of pollution.
Source: Al Jazeera and news agencies