The attacks began at 1 p.m. on Monday, Korea's Independence Day, which marked Korea's movement against Japanese colonial rule 91 years ago.
Login attempts were unsuccessful in the afternoon.
About 30 out of 33 posting boards were paralyzed by 1:30 p.m.
Online security officials estimated that more than 10,000 Korean netizens participated in hacking into the site.
The attacks were triggered by comments praising Russians for the killing of a Korean student in Moscow.
Some postings claimed that the killing was not racially motivated and Russians were doing "good deeds" after the event occurred.
2ch saw postings claiming that Kim Yu-na won the gold medal in the Vancouver Olympics thanks to judges bribed by Korea. Such anti-Yu-na comments increased when Japan's Mao Asada lost to Kim and had to settle for a silver medal.