By Hongji Kim

SEOUL (Reuters) - South Korean police arrested on Thursday at least 16 people who entered a building housing the Japanese embassy in Seoul during a protest against Tokyo's release of water from the Fukushima nuclear plant.

The protest came the day Japan began releasing treated radioactive water from the wrecked power plant into the Pacific Ocean, a polarising move that prompted fresh, fierce criticism from around the region.

The group, made up mostly of young people, reached the eighth floor, where the embassy is located, and hung banners condemning the release.

"The sea is not Japan's trash bin," read one of the banners. "Stop releasing contaminated water at once."

A Reuters photographer on the scene saw police officers physically carry and drag protesters from the building and bundle them into a bus.

A police officer at the Jongno police station in the capital said 16 people were arrested on charges of trespassing, accusing them of trying to break into the embassy.

About 50 people had gathered outside the embassy for the protest.

South Korea's government has said that it sees no scientific problems with the water release, but called on Tokyo to be transparent during the process.

"What's important now is whether Japan, as it promised to the international community, strictly follows the scientific standards and transparently provides information," Prime Minister Han Duck-soo told a briefing.

(Reporting by Hongji Kim, Additional reporting by Soo-hyang Choi; Writing by Josh Smith; Editing by Clarence Fernandez and Christopher Cushing)