"Nearly 80 representatives of a growing and truly independent civil society have been arrested, convicted and sentenced to lengthy prison terms in summary, secret trials," Powell said in a statement.
"Their only crime was seeking basic human rights and freedoms."
Omitting the Cuban leader's title and first name, Powell urged "Castro" to free these "prisoners of conscience." Powell said the United States and the international community will be unrelenting in its insistence that "Cubans who seek peaceful change be permitted to do so."
Powell's comments came as the last sentences in the crackdown on opposition were announced Thursday, bringing to 1,454 the number of years that 75 dissidents were collectively ordered to serve on charges of collaborating with U.S. diplomats.
"There has never been anything similar to this in the history of Cuba,'' said veteran activist Elizardo Sanchez, whose Cuban Commission on Human Rights and National Reconciliation has monitored the arrests and subsequent trials.
"We call on all democratic governments and organizations of the world that have not done so already so to openly reject this wave of repression," read a letter signed by five leaders of the local opposition, including Sanchez.
"We direct this call in particular to our brother countries in Latin America, which up to now have not spoken out in this needed censure of the only totalitarian regime" in the region, added the letter, also signed by veteran activists Gustavo Arcos, Vladimiro Roca, Rene Gomez and Felix Bonne.
Governments and human rights groups around the world have condemned the crackdown, which began with a massive roundup on opponents on March 18. The subsequent trials -- none of which lasted more than a day -- resulted in sentences ranging from 6 to 28 years.