police. 
It brought together various groups, including trade unions opposed to the president's business-friendly policies.
Others were protesting against the imposition of state-approved history schoolbooks, which they say whitewash South Korea's past dictatorships.
Many marchers chanted "Park Geun-hye, step down''. Security forces fired tear gas when some of them tried to break through police barricades as they moved towards the presidency.
Park Geun-hye, South Korea's first female president, was elected two years ago.
She is pushing through controversial plans to make labour markets more flexible by giving employers more leeway in dismissing workers.