Prosecutors said Tuesday they have indicted a former chief of South Korea's second-largest umbrella labor group on charges of holding unauthorized rallies.

The Seoul Central District Prosecutors' Office said Shin Seung-cheol, former head of the Korean Confederation of Trade Unions (KCTU), is accused of holding six illegal rallies from 2013 to 2014.

Prosecutors said the 52-year-old, who led the union from August 2013 until December 2014, gathered some 3,000 protesters against rail privatization in central Seoul in December 2013, after reporting that the estimated number of demonstrators was 20.

Under South Korean law, those wanting to hold a rally should first report related information to the authorities.

Shin is also suspected of obstructing traffic by illegally occupying streets with some 2,500 protesters during another rally, according to prosecutors. (Yonhap)