HAVANA -- Cuban agents rounded a leading independent journalist and several government critics Thursday, part of a major crackdown spurred by allegations of dissidents conspiring with U.S. diplomats.

The detention of Raul Rivero indicated security agents were beginning to focus on higher-profile dissidents. The crackdown is the latest in a series of moves that have further strained U.S.-Cuba relations.

"He is only a man who writes, he is not a politician," Rivero's wife, Blanca Reyes, said after agents led her husband from their home and sped away in a sedan.

"He knew they would come for him," said Reyes. "But until then he was informing the entire world what was happening here."

Earlier, the agents detained several people at a home where they were fasting to demand the release of a leading political prisoner.

The arrests raised the number of detentions during three days of sweeps to at least 59, the nongovernmental Cuban Commission on Human Rights and Reconciliation said.

State security agents Thursday evening also were conducting a search at the home of Hector Palacios, a leading organizer of the Varela Project, a movement aimed a bringing reform to Cuba.

The group was investigating reports of at least 15 other arrests.

"We don't know how far this crackdown is going to go," said Elizardo Sanchez, a leading rights activist. "The Cuban government wants to silence the dissident movement. But that is not possible."

There are believed to be at least hundreds of dissidents in Cuba. The government says many are conspiring with American diplomats in Cuba to drum up opposition to the socialist state.

On Wednesday, the Bush administration denounced the arrests as "an appalling act of intimidation."

Washington's chief diplomat in Havana, James Cason, has repeatedly met in public with opposition leaders in what he says is an effort to encourage democracy on the island. But his appearances with dissidents have further strained U.S.-Cuba relations.

In announcing the first wave of arrests Tuesday, the Cuban government also said it was restricting travel on the island by American diplomats.

Cuba has said it will put the dissidents on trial, raising fears they will be prosecuted under a law that carries sentences of up to 10 years and makes it a crime to publish "subversive" materials provided by the U.S. government.

The United States distributes shortwave radios and a wide range of books and pamphlets throughout Cuba aimed a promoting American culture, democracy and human rights.

Relatives of Marta Beatriz Roque confirmed she was among a small group of people detained around 6 a.m. at the home in Havana where they had been fasting since March 11.

The dissidents had pledged to consume only liquids -- yogurt, soup and juices -- until fellow dissident Dr. Oscar Elias Biscet is released from jail. Biscet was imprisoned in December during a protest in nearby Matanzas Province.