The Japanese government must face its past and directly address those victimised by its soldiers, said 88-year-old Lee Ok-Sun, who said she was forcibly taken to China as a teenager to work in Japan's military-run brothels.
Lee stressed that survivors like her are angry over the December deal, which she said was struck without consulting them.
"There are living victims," she said through an interpreter at a gathering of more than 100 people in Tokyo hosted by Japanese supporters.
Lee is among tens of thousands of women in Asia, mostly from Korea, who were forced to provide sex to Japanese soldiers during World War II.
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She is visiting Japan with Kang Il-Chul, another former sex slave who was absent from the event due to fatigue.
The plight of the ageing so-called "comfort women" is a hugely emotional issue that has for decades marred ties between Seoul and Tokyo, which ruled the Korean peninsula harshly from 1910-1945.
But the two nations reached a deal in December, under which Tokyo offered an apology and a one-billion yen ($8.5 million) payment to surviving South Korean women -- an agreement both nations described as "final and irreversible".
They also have bitterly complained that neither Seoul nor Tokyo consulted the women about the agreement before concluding it.
Lee said victims want Japan to admit its past and issue an official apology while they are alive.
The Japanese government has long maintained that issues related to Japan's colonisation of the peninsula and the war were settled in a 1965 agreement which saw Tokyo establish diplomatic ties with Seoul and make a payment of $800 million.
"Can you tell me... Not to protest to the Japanese government?
"I want you to imagine the pain and anger that we experienced.