President Yoon Suk Yeol celebrated the opening of the Yongsan Children's Garden on what was formerly part of the US' Yongsan Garrison military base on Thursday.

The 300,000-square-meter site is open to the public for the first time in 120 years, as the central Seoul site had been used for military facilities by Qing troops in the late 19th century, the Imperial Japanese army during the colonial era and US Forces Korea after Korea's liberation in 1945.

The ceremony to celebrate the transformation of the southwestern part of the Yongsan Garrison site near the presidential office in Yongsan-gu, Seoul, was attended by over 200 people including 74 children, as well as their parents, government officials and lawmakers on the eve of Children's Day, which Korea celebrates as a national holiday.

The garden features a 70,000-square-meter field, a library, a sports ground where children at the age of 12 or younger can play soccer and baseball, a coffeehouse and cottages that will operate as a history museum. There are also family-friendly facilities for visitors to breastfeed, change babies' diapers or borrow strollers or wheelchairs.

"Seoul has yet to have a decent outdoor green grass field on which children can run, and this was behind the idea of transforming the field and the surroundings into a park for children," Yoon said in a speech Thursday.

Yoon added Thursday that the relocation of the presidential office to the Yongsan neighborhood from Cheong Wa Dae near Gyeongbokgung has enabled the swift transition of the site into a garden.

Yoon outlined his plan to open the park on the former US military base site immediately after he won the presidential election in March 2022. Yoon was inaugurated in May 2022.

The space is part of the 2.43-square-kilometer site used for Yongsan Garrison. Under the military base relocation agreement signed between Korea and the US in 2004, the US began the process of returning of the site in 2020. It has since returned over 30 percent of the total. Last year, the US returned 584,000 square meters of land -- or 24 percent -- to the South Korean government, including the site where the Yongsan Children's Garden is.

On the sidelines of the ceremony, however, environmental activists flocked to the garden to oppose the presidential office's plan to open the site to the public, citing health concerns due to toxic chemical substances -- such as total petroleum hydrocarbons, arsenic and lead -- that remain in the soil and groundwater of the former military base.

The government played down such concerns in a statement earlier in April, saying the country had added a 15-centimeter-thick layer of dirt to the entire garden site.

Only visitors who make reservations are permitted to enter Yongsan Children's Garden. Reservations must be made online five days prior for Korean nationals and 10 days prior for foreign nationals. Repeat visitors will not require a reservation.