The sit-in by the New Politics Alliance for Democracy (NPAD) comes amid deadlock in parliament over plans to set up an independent inquiry into the April 16 sinking, which claimed some 300 lives.
The ruling Saenuri Party has rejected a call for a consultative body allowing victims' families a role in discussing legislation needed for such an inquiry, saying this would amount to allowing crime victims to take the law into their own hands in breach of the constitution.
"There can be no state if people's lives are neglected," NPAD parliamentary floor leader Park Young-Sun told a rally outside parliament as the sit-in got underway.
"We will fight on with the victims' families... Until the Saenuri Party and President Park Geun-Hye accept the demand" for an independent inquiry, he said.
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The row over the inquiry has paralysed all parliamentary work, including bills aimed at boosting the nation's sluggish economy.
The tragedy, blamed by many on regulatory failings and official incompetence, plunged South Korea into deep mourning and soul-searching. Most of the dead were students at the same high school.
Fifteen of the boat's crew members are currently on trial, including the captain and three senior officers who are accused of "homicide through wilful negligence" -- a charge that can carry the death penalty.