About 80,000 people were expected to turn up for the downtown rallies that were to stretch into the evening, according to an official at the Seoul Metropolitan Police Agency, who didn't want to be named, citing office rules.
The marches, organized by labor, civic, and farmers' groups, brought together protesters with a diverse set of grievances against the government of conservative President Park Geun-hye, including her business-friendly labor policies and a decision to require middle and high schools to use only state-issued history textbooks in classes from 2017.
A Seoul court had issued an arrest warrant for Han over a failed court appearance, after he was indicted for his involvement in organizing a May protest that turned violent.
Demonstrators, many of them wearing masks and carrying banners, occupied a major downtown street and began marching between tight perimeters created by police buses and portable walls, intended to block them from entering large roads leading to the presidential Blue House.
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This was probably the largest crowd seen in a demonstration in Seoul since 2008, when people poured onto the streets to protest the government's decision to resume US beef imports amid lingering mad cow fears, said the Seoul police official.
Labor groups have been denouncing government attempts to change labor laws to allow larger freedom for companies in laying off workers, which policymakers say would be critical in improving a bleak job market for young people.
Critics say that the state-issued history textbooks, which haven't been written yet, would be politically-driven and might attempt to whitewash the brutal dictatorships that preceded South Korea's bloody transition toward democracy in the 1980s.
South Korean police in May detained over 40 people when protests over the government's labor policies and the handling of a year-old ferry disaster spiraled into violence, leaving several demonstrators and police injured and many police buses damaged.