Students threw Molotov cocktails at police during a march on Saturday by thousands of South Koreans protesting against the government’s economic reforms.
More than 5,000 unemployed workers, students and labour activists joined the demonstration, which was smaller than organisers had predicted.
About 50 masked students hurled petrol bombs at police when they were prevented from marching out of Yonsei University in western Seoul.
There were no reports of any injuries. Three students were injured in a separate clash earlier in the day.
Police are also investigating a website which contains instructions for manufacturing a more dangerous type of Molotov cocktail, which it is feared could be used by rioters.
Molotov cocktails were often used in protests against military-backed governments in the 1980s, but had largely disappeared from use.
But in recent protests, demonstrators have used them against police attempting to break up demonstrations.
Their return has led to police launching new units to counter the devices.
Saturday’s protest was called by the Korean Confederation of Trade Unions to highlight opposition to the government’s economic reforms, including restructuring of companies and job losses.
Organisers has hoped as many as 20,000 protesters would attend.
The unemployment rate in February was the highest level in a year, with more than a million people out of work.