The protesters chanted and held placards in front of Tokyo's embassy in Seoul. With a local ban on gatherings larger than 10 people, police periodically dispersed crowds but did not stop the event from taking place.
Japan's government said last week it will release more than 1 million tons of treated water from the Fukushima site in stages starting in about two years.
Seoul has strongly rebuked the decision, with the foreign ministry summoning the Japanese ambassador and President Moon Jae-in ordering officials to explore petitioning an international court.
U.S. climate envoy John Kerry on Sunday (April 18) said he believed Japan had made the decision in a transparent manner and would continue to follow due procedures.