Police raided the local branch offices of an umbrella labor union Wednesday as part of their ongoing probe into a violent anti-government rally held last month.

The Seoul Metropolitan Police Agency raided three regional offices of the plant construction labor union affiliated with the Korea Confederation of Trade Unions in South Chungcheong Province and one in South Jeolla Province.

The members of the union are suspected of preparing iron pipes for the rally, as well as brandishing them against police officers during the protest, according to the police.

Earlier in November, tens of thousands of demonstrators took to the streets of downtown Seoul to protest the government's decision to adopt state history textbooks and reform the labor market. The rally later turned violent as some protesters brandished the metal pipes and police fired water cannons at them.

The raid took place ahead of a massive rally planned for this weekend, after an association of liberal civic groups vowed to go ahead with the protest in central Seoul on Dec. 5, despite a prohibition order from the police. (Yonhap)