PRESIDENT OBAMA’S homage to Nelson Mandela on Tuesday was moving and heartfelt. He celebrated a “great liberator” who demonstrated the power of words, ideals and actions to change history. But the president added an awkward footnote to his tribute in Soweto by stopping to shake hands with Raúl Castro, a man whose regime, led for a half-century by his brother Fidel, has bashed heads and broken arms to stifle freedom.

A handshake is a gesture, in this case one freighted with symbolism that cannot be ignored. Tuesday marked the 65th anniversary of the adoption of the Universal Declaration of Human Rights by the United Nations. On the streets of Havana and other cities, a crackdown on civil society was underway. Mr. Castro’s goons showed that they have not lost their taste for violence and coercion to extinguish even the slightest protest or expression of free will.

We called attention two days ago to the work of Antonio Rodiles, a democracy activist who announced his intention to hold a human rights conference in Havana on Tuesday. In a letter to Mr. Castro, he described a litany of harassment and abuse directed at him by Cuba’s security forces and thugs under their control, who threatened retaliation if the conference went ahead. Sure enough, the authorities followed through on their threats. The home of Mr. Rodiles was cordoned off, and most of those who came to participate in the conference were barred from entering. Then Mr. Rodiles and several colleagues were arrested Wednesday.

According to a Reuters dispatch, about 20 members of the dissident group Ladies in White “were pounced upon and quickly shoved into waiting vehicles by security personnel and government supporters” when they arrived Tuesday at a busy Havana intersection. The Miami Herald reported that the group’s leader, Berta Soler, and her husband, former political prisoner Angel Moya, “were hauled off by plainclothes police as they headed” to the planned protest. Security officials also blocked the telephones of several dissidents in an apparent effort to silence news of other arrests. The popular blogger Yoani Sánchez tweeted Tuesday morning, “Like in a bad horror movie, I am losing communication with . . . activists.”

Elsewhere on the island, there were reports that independent journalists, filmmakers and writers were arrested. The Herald reported that police left 16 dissidents bleeding and that six others were arrested when they raided the home of Roger Curbelo, a member of the opposition Christian Liberation Movement in the town of Puerto Padre. The movement was once led by Oswaldo Payá, the dissident who was killed last year in a suspicious car wreck.

While Mr. Obama was shaking hands with Mr. Castro, courageous people attempting to uphold Mr. Mandela’s ideals were suffering beatings and arrests. The president ought to follow his handshake with a loud and unambiguous salute to the real champions of human rights — those fighting for it on the streets of Cuba.