Amid mounting controversy over the alleged connection between one of Korea's major national labor union umbrella organizations -- the Korean Confederation of Trade Unions -- and North Korea, police investigating illegal activities at construction sites raided three chapter offices that are under the umbrella organization, Tuesday.

At around 8:30 a.m. Tuesday, the Seoul Metropolitan Police sent investigators to three local chapter offices of the KCTU's construction union, the Korean Construction Workers Union, in Seoul, to secure evidence related to charges of forcing companies to hire union members and extorting money and valuables from companies.

The raids are part of a special crackdown on allegedly illegal union activities at construction sites that began last December. The police plan to continue investigating whether officials at the top of the union were involved in coercion and extortion.

On the same day, the North Jeolla Provincial Police also raided the offices of the Tower Crane union under Korea's other major umbrella labor union organization, the Federation of Korean Trade Unions, as well as the KCTU construction union's office in Jeonju, North Jeolla Province.

In addition to evidence of illegal activities at construction sites, the KCTU has become a center of controversy as the National Intelligence Service and the police have reportedly found a number of pieces of evidence -- instructions and a pledge of allegiance -- that connect the union with North Korea. The evidence was found during a January raid of the KCTU headquarters over charges of violating the National Security Act.

The instructions, allegedly sent from North Korea, instigate people to call for the resignation of President Yoon Suk Yeol, creating an atmosphere for his impeachment, according to local media reports, Monday.

In particular, the NIS said that after the Itaewon crowd crush disaster on Oct. 29 last year, North Korea even sent specific slogans to use on handheld signs at a related rally saying, "people are dying," "resignation is rightful remembrance." At the time of the nationwide Cargo Truckers Solidarity Union strike in November and December last year, North Korea also sent an order saying, "Instigate public anger with all-out efforts and solidarity," the NIS claimed.

A number of pledges of allegiance to North Korea written by KCTU officials were also reportedly found in the confiscated materials. The pledge of allegiance is said to contain a phrase praising North Korean leader Kim Jong-un.

Han Sang-jin, a spokesperson for the KCTU, strongly refuted every allegation and accused the NIS and conservative media outlets of suppressing the union. Han stressed that the KCTU never used slogans that call for the resignation of Yoon at the memorial rally for the Oct. 29 crowd crush in Itaewon.

“The KCTU discusses, decides and executes everything together, but demanding the resignation of the President Yoon Suk Yeol has never been on the table. If anyone has proof of us using handheld signs, pickets or leaflets calling for Yoon’s resignation at the rally related to the 10.29 crowd crush, I ask them to come forward,” said Han via phone to The Korea Herald.

"The behavior of the NIS and the conservative media outlets, which even mock the victims and their bereaved families who are still in pain, has not changed," he criticized.

Meanwhile, the ruling People Power Party demanded countermeasures to eradicate pro-North Koreans in the country.

“The KCTU has clearly profited the enemy (North Korea) for years. A thorough investigation, severe punishment and taking of responsibility must follow, regardless of who it is or what organization it is," said Rep. Sung Il-jong at a ruling party meeting held at the National Assembly on Tuesday.

Rep. Chung Woo-taik, deputy speaker of the National Assembly, depicted the situation as “appalling,” and claimed that the impetuous decision of the previous administration to abolish the NIS’ right to conduct anti-communist investigations had resulted in this situation.

"It is urgent to come up with special measures to prevent espionage forces from spreading themselves," said Chung on Facebook, Tuesday.