Prosecutors raided the headquarters of the Korean Government Employees' Union in Ilsan, Gyeonggi Province, and seized the server of its website early Friday in the first step of an investigation into the union's alleged interference in the presidential election last year.
Seoul Southern District Prosecutor's Office has launched the probe last Monday after conservative civic group Young's Liberty Union accused the union of allegedly conducting an illegal online election campaign in support of Democratic Party candidate Moon Jae-in.
Civil servants are banned from participating in election activities or expressing political preferences in public.
The conservative group insisted that the union failed to maintain political neutrality by posting messages such as "(We) support people's candidate Moon Jae-in," or "Let's cast all the votes to No. 2 candidate Moon Jae-in."
The ruling Saenuri Party supported the prosecution's decision.
"If the prosecution found or attained leads pertaining to the KGEU's allegations, conducting an investigation is reasonable," said Saenuri floor spokesman Kim Tae-heum.
Last week, the ruling party alleged that the DP had agreed to guarantee six advantageous regulations for the union in return for election support, and urged the prosecution to launch an investigation immediately.
"Moon made a policy pact with the union on Dec. 7 last year to receive support, and the KGEU published the pact on its website," the ruling party said in a statement.
Opposition parties criticised, claiming that the prosecution's political intention was clear.
"This is absolutely a strategic probe ordered by authorities to cover up people's anger over the government agencies' alleged interference in the presidential election and water it down," DP spokesperson Park Yong-jin said.
Since the probe, the KGEU has protested the probe by organising an emergency team and initiating a 24-hour, weeklong protest on Wednesday.
"From the accusation to investigation, we cannot help but feel that everything was carried out according to a script. The government is using the union to cover the state institution's interference in the presidential election," said the KGEU president Kim Jung-nam.
A total of 288 civic groups joined the protest on Thursday.
"As the allegation of the state institution's interference in the election is expanding, the government and the ruling party are fabricating the truth to escape the situation," they said.
The Ministry of Security and Public Administration said it would consider taking measures including punishment for those found guilty in the investigation.