South Korea's crackdown on protests it sees as unlawful is materializing amid escalating tensions between the conservative Yoon Suk Yeol administration and local labor unions voicing disapproval of Yoon's union-busting drive.

On Thursday, the police detained three demonstrators at around 8:50 p.m. in front of the Supreme Court of Korea in Seocho-gu, Seoul, for staging a protest without providing proper notification. It was Seoul's first crackdown on protesters since the COVID-19 outbreak.

About 80 members of a construction union held what it labeled "a nighttime cultural festival." As the festival seemed to turn violent, with chants criticizing the Yoon administration, some 400 police launched a crackdown operation after warnings to stop the chants.

Under Korean rules, an organizer of an assembly or demonstration should notify the police authority before the scheduled date. The rule, however, does not permit protests before sunrise or after sunset unless under exceptional circumstances.

The rule discouraging nighttime protests "as far as the nature of such event makes it inevitable" has been largely deemed unconstitutional.

In Thursday's case, the organizer did not notify police of the assembly, in hopes that holding "a festival" at night would not cause them any trouble. But police warnings came and the crackdown followed on the logic that the chants at the event deemed it an unlawfully organized protest.

This comes as the latest development of worsening relations between the Yoon administration and labor unions.

Tuesday's meeting of ruling party and government officials at the National Assembly hinted at a ban on nighttime protests with new legislation.

Yoon Hee-keun, chief of the Korean National Police Agency, pledged to fortify police forces to "prevent physical violence at the scene of the protests, as well as traffic congestion and noise due to protests," in a letter to national police. Police also on Thursday resumed a field training exercise to crack down on unlawful protesters for the first time in six years.

Meanwhile, labor unions staged a two-day demonstration last week, which involved sit-ins on streets in central Seoul throughout the night.

Yoon's "zero-tolerance" stance against labor union activities labeled as unlawful, including lack of transparency in accounting, has triggered backlash from unions across the nation, including the self-immolation of a union member on May 1 in Gangwon Province.