President Yoon Suk-yeol receives a briefing from the military in the underground bunker of the new presidential office in Yongsan, Seoul, on Tuesday, in this photo provided by his office. (Yoon's presidential office)

Yoon Suk-yeol is set to be sworn in as South Korea's new president Tuesday, marking the start of a tough battle to avert an economic crisis, win the cooperation of an opposition-controlled parliament and rein in an increasingly menacing North Korea.

Yoon kicked off his five-year term at midnight in the underground bunker of the new presidential office building in Yongsan by receiving a briefing from the Joint Chiefs of Staff.

"President Yoon Suk-yeol expressed his appreciation for the hard work of our troops who are dedicating themselves to defending the land and protecting the people's lives and property night and day, and emphasized they maintain a firm military readiness posture at this time when the security situation on the Korean Peninsula is grave," the presidential National Security Office said.

To usher in the new administration, a bell-ringing ceremony was held at the stroke of midnight in downtown Seoul.

Twenty representatives selected from the general public, including a naturalized citizen, a space scientist and a North Korean defector, rang the bell 33 times at Bosingak Pavilion.

The formal inauguration ceremony is scheduled for 11 a.m. at the National Assembly Plaza.

More than 40,000 people will gather at the inauguration ceremony, including foreign envoys, such as US second gentleman Douglas Emhoff and Chinese Vice President Wang Qishan, a gathering size that was impossible until recently due to COVID-19 restrictions.

Yoon will take his oath of office, deliver a speech around keywords such as freedom, human rights, markets, fairness and solidarity, and head straight to the new presidential office he fought hard to launch as a demonstration of his will to draw closer to the public.

Cheong Wa Dae, the former presidential office built on a majestic compound at the foot of a mountain, was viewed by Yoon as a "symbol of imperial power."

Yoon takes over at a time when South Korea is struggling to deal with economic challenges stemming from the pandemic, the war in Ukraine and other factors resulting in the phenomenon of "three simultaneous highs" in inflation, interest rates and exchange rates.

The incoming government has championed "economic security" amid the growing competition between the United States and China to secure supply chains in batteries, semiconductors and other key sectors.

The threat of North Korea's nuclear and missile programs looms larger than ever, as the communist nation appears set to carry out its seventh nuclear test as early as this month, shortly after North Korean leader Kim Jong-un threatened to proactively use nuclear weapons, rather than possessing them only as a war deterrent if anyone attempts to violate the country's "fundamental interests."

Both economic security and North Korea are expected to feature high on the agenda of Yoon's first summit with US President Joe Biden in Seoul on May 21.

Biden's visit, set for May 20-22, will come only 10 days after Yoon takes office, and their planned meeting will mark the earliest-ever Korea-US summit to take place following a South Korean president's inauguration.

Yoon also faces the daunting task of repairing deeply fractured ties with Japan.

During the campaign, he indicated his will to build a future-oriented relationship with the neighboring country despite their unresolved disputes over wartime sex slaves, forced labor and territory stemming from Tokyo's 1910-45 colonial rule of the Korean Peninsula.

Last month, he sent a policy consultation delegation to Japan with a letter for Japanese Prime Minister Fumio Kishida.

With China, Yoon faces a tough balancing act, as Beijing's cooperation is key to reining in North Korea's nuclear ambitions and maintaining a robust bilateral trade relationship, while the president has pledged to deploy additional units of the US THAAD antimissile system in South Korea -- a major irritant for Beijing -- and seek South Korea's gradual entry into the Quadrilateral Security Dialogue, a US-led forum regarded as countering China's rise.

On the domestic front, Yoon faces a hostile National Assembly controlled by the main opposition Democratic Party.

With 168 out of 300 seats, the DP has delayed the parliamentary confirmation process for Yoon's Cabinet nominees, forcing the new government to hold its first Cabinet meeting this week with several members of the outgoing administration.

The legislative hurdles were demonstrated clearly in the transition team's decision to postpone its government reorganization, including whether to keep Yoon's campaign promise to abolish the Ministry of Gender Equality and Family.

With the June 1 local elections only weeks away, the new government also wants to avoid a scenario where Yoon's ruling People Power Party loses the elections or the seven parliamentary by-elections being held concurrently so early in its term.

On Sunday, Yoon's election rival, Lee Jae-myung, declared an earlier-than-expected comeback to politics by announcing a run for one of the parliamentary seats up for grabs in the by-elections.

Yoon's popularity ratings have been just around 50 percent, one of the lowest levels for a president-elect, which underscores the deep political divisions in South Korean society.

Until a little over a year ago, Yoon, 61, was the nation's top prosecutor with a reputation for conducting high-profile investigations into powerful figures, such as former President Park Geun-hye and former Justice Minister Cho Kuk.

The probes earned him the wrath of first the conservatives and then the liberals. In the end, it was the conservatives who summoned him into politics and elected him into the top office, marking the first time the government had changed hands between liberals and conservatives after a single, five-year term.

The gap between Yoon and his main rival was a mere 0.73 percentage point. (Yonhap)