On his 100th day in office, South Korean President Moon Jae In will put himself in the line of media fire, ready to tackle tough topics ranging from North Korea's missile crisis to the controversial phasing out of nuclear power to impending tax hikes.

The press conference, to be held on Thursday, will be an opportunity for a "candid conversation" with the media, said the presidential Blue House.

Such is the open-door approach of the popular liberal leader.

Indeed, a week after his election, Mr Moon broke security protocol at a memorial and hugged a woman whose father was among hundreds killed in a crackdown against a major democratic movement.

The widely applauded gesture on May 18 showed his ability to reach out and connect with the people - a stark contrast to his authoritarian predecessor Park Geun Hye.

It also helped propel his approval rating to 87 per cent - the highest in history, according to pollster Gallup Korea.

Since his inauguration on May 10, the 64-year-old human rights lawyer-turned-politician has worked swiftly to overhaul the government, break internal hierarchy, and restore faith in a nation scarred by the massive corruption and influence-peddling scandal that brought about Park's downfall.

Mr Moon's high approval rating - 78 per cent as of Aug 11, according to the latest Gallup poll - is testament to his popularity. To mark his 100th day, Korea Post will be issuing five million stamps featuring Mr Moon in different phases of his life, from childhood to marriage to his inauguration.

Housewife Kim Su Hyun, 40, whose entire family voted for Mr Moon, is full of praise for him. "He listens humbly to the people, and he is always open to communication. Compared to the frustration we felt with Park, he brings heartfelt enthusiasm."

Analysts said Mr Moon has done well to reopen public communication channels in the past three months, and that he has had some success in pushing for reforms. He has launched probes into corruptive practices, unveiled a slew of welfare programmes, and is working to move his office from the secluded Blue House to a more accessible government building so as to be closer to the people.

But as his honeymoon period draws to an end, experts warn of major challenges and tough choices ahead - from pushing his policies through in Parliament where his Democratic Party does not have a two-third majority to approve Bills, to resolving the dilemma he faces in trying to engage a North Korea that is bent on raising tensions in worsening rhetoric with United States President Donald Trump.

Wilson Centre global fellow Jean Lee said Mr Moon, who was sworn in a day after his election and had no luxury of a transition period like his predecessors, "showed very quickly" that he is action driven.

"President Moon was smart about making himself accessible and transparent, which is what South Korea needed from their president," she told The Straits Times. "And he has shown he is open to hearing a wide range of opinions, from liberal to conservative, but can make practical decisions."

Asked to rate Mr Moon's 100-day performance, Dr Lee Seong Hyon from think-tank Sejong Institute gave him nine out of 10 for domestic affairs, and seven for foreign policy. "Moon regained the Korean public's trust on the government. He and his wife are very talented in terms of public diplomacy. I think he is genuine when he empathises with underprivileged people."

Last month, Mr Moon unveiled a five-year road map to achieve his vision of creating a "people's country" where unfair privileges and collusive links - which led to Park's scandal - are removed.

Key goals include reforming the police, prosecution and spy agency, toughening anti-corruption measures, beefing up the country's defence capability, promoting fair competition among businesses, creating 810,000 public service jobs, raising the minimum hourly wage to 10,000 won (S$12) from the current 6,470 won, and converting part-time jobs to regular ones that offer job security.

It may sound all nice and rosy, but critics and media have accused Mr Moon of pandering to populism and questioned how the government can finance so many projects, given South Korea's stagnating economy exacerbated by China's economic retaliation against the deployment of a US missile shield.

The new measures will cost an estimated 178 trillion won over five years, over half of which officials said can come from the existing Budget, after making some adjustments. The rest will have to come from an inevitable increase in taxes - a move that has raised many eyebrows and drawn concern about the financial burden on the public.

Dr Bong Young Shik from Yonsei University warned that plans to raise the minimum wage next year will adversely impact small and medium-sized enterprises as labour costs soar. This could shake Mr Moon's support base and breed discontent, he said.

Seoul National University professor Lee Jae Min said the speed of change raises concern, as major policies are rolled out too quickly without consultation with key interest groups.

Mr Moon's decision to shut the country's oldest nuclear plant and suspend the construction of new plants, while in line with his election campaign, triggered protests citing economic concerns. The decision is now under review, which Mr Moon's aides said reflects his willingness to listen to public opinion.

Prof Lee said: "I think he is cruising smoothly overall, but at the same time, he is now bringing Korea into uncharted terrain for many issues... Concerns are growing because of the speed and depth of his reform measures."

Analysts also pointed out that Mr Moon's pro-rapprochement has yet to score big on the foreign policy and national security fronts. He has neither managed to thaw frosty ties with China, nor was he able to help ease growing tensions between North Korea and the US.

Dr Lee said Mr Moon picked a good foreign minister - United Nations veteran Kang Kyung Wha - but needs more strategists to help him overcome tough challenges ahead as he grapples with pressure from China, the US and North Korea. "If Moon alone insists on an engagement policy with North Korea, his diplomatic manoeuvring room will become smaller and smaller. So if Moon ever fails, it will be on the foreign policy frontier," he said.