A University of Tokyo graduate school has criticized Hong Kong authorities for charging one of its students for violating the territory's national security law.

The university's Graduate School of Public Policy said in an English statement on Monday that PhD student Au Nok Hin was arrested as part of a broad crackdown, the day after he returned to Hong Kong. He had been studying at the school.

Au is one of the 47 defendants who went on trial in Hong Kong on Monday, on a charge of conspiracy to commit subversion.

The statement says his passport, computer and smartphone are still confiscated, and are all vital for his academic work.

It says this infringes on "Mr. Au Nok Hin's unwavering determination to concentrate on his PhD project, which has the full support of the Graduate School."

The statement continues, "We have no doubt that he will be cleared of his charges in the very near future and eagerly await his return to our academic community in person where he belongs."

Au is a former member of Hong Kong's Legislative Council. He is accused of calling for an unofficial primary poll to choose pro-democracy candidates for a Council election, which was due last September but was postponed.

Others facing the same charge include former University of Hong Kong associate professor Benny Tai.