The U.S. arm of Amnesty International, the global human rights group, broke the law by threatening its own employees, a National Labor Relations Board judge has ruled.

Managers at Amnesty International USA violated the law that protects employees’ right to organize for improved working conditions, Administrative Law Judge Michael Rosas wrote in a decision issued Tuesday.

According to the ruling, last year a group of unpaid interns, with support from some of Amnesty’s unionized permanent employees, drafted a petition to their supervisor asking to be paid. “Amnesty International’s commitment to human rights should be proven from within first,” they wrote,...