A major umbrella labor union slammed the government on Monday for launching an investigation into an illegal mass rally it held over the weekend.

The Korean Confederation of Trade Unions (KCTU) took to the streets of Seoul's central Jongno Ward on Saturday to demand better working conditions and a higher minimum wage.

The rally, which defied bans by the Seoul metropolitan government and the police, drew an estimated 8,000 people at a time when the capital area has seen a sharp increase in the number of COVID-19 cases.

"(The police's) launch of a special investigation unit and (pledge of) a strict response after shutting their eyes, ears and mouths to the desperate pleas of workers is telling of the remainder of the (current administration's) term," a KCTU official said during a press conference held at the organization's headquarters.

"It's obvious that they are trying to isolate and kill the KCTU," he said.

KCTU officials said they share the government's concern about the spread of COVID-19 but disagree with its response.

They accused the government of discrimination, saying it has allowed sports events and concerts but not rallies even though they are all held outdoors.

"Where is the freedom of political thought and expression, which is the foundation of democracy, and the freedom of assembly that supports this?" they asked.

Holding up photos to illustrate their point, the officials claimed police did not respond as strictly to the large number of supporters who gathered outside the venue where former Prosecutor General Yoon Seok-youl, a leading opposition presidential hopeful, announced his presidential bid last week.

The KCTU vowed to press ahead with a general strike in November. (Yonhap)