International rights group Amnesty International said on Monday it would close its Hong Kong offices because a China-imposed security law now made it "effectively impossible" for rights groups to work freely without the risk of reprisals.

Anjhula Mya Singh Bais, the chair of Amnesty’s international board, said in a statement that the two offices would close by year-end, noting an intensification of a crackdown that has forced at least 35 groups to disband under the law this year.

"This decision, made with a heavy heart, has...