(Adds State Department condemnation, other details)
By Daniel Trotta
HAVANA, Dec 30 (Reuters) - Cuban police detained several dissidents on Tuesday and thwarted an unauthorized political protest, dissidents said, in the first major test of U.S. President Barack Obama's policy shift toward normalizing relations with the communist-ruled island.
The U.S. State Department condemned the detentions, which marked the most significant crackdown on the opposition since Cuba and the United States agreed on Dec. 17 to restore diplomatic ties and put behind more than five decades of hostility.
About 12 opponents were taken away by police, including the husband of opposition blogger Yoani Sanchez, while several others were told not to leave their homes as police parked outside, said Elizardo Sanchez, leader of a dissident human rights commission that monitors such detentions.
Other dissident leaders reported multiple detentions or that activists were ordered to stay at home. Yoani Sanchez's website 14ymedia.com reported she was under virtual house arrest.
The detentions stopped a planned open microphone event at Havana's Revolution Square, near the government headquarters.
Event organizer Tania Bruguera, a performance artist, was missing and her associates presumed she, too, had been detained. Bruguera had vowed to go ahead with the event even after Cuban officials denied her a permit.
The event flopped, with only 15 participants, surrounded by a phalanx of reporters. A parallel event in Miami called by Cuban exiles drew 50 people.
Cuba had called the open microphone event a "political provocation," and it was unclear how long the dissidents would be held. Cuba typically holds dissidents for several hours and releases them.
"Freedom of expression remains core of U.S. policy on Cuba; we support activists exercising those rights and condemn today's detentions," tweeted Roberta Jacobson, the U.S. Assistant Secretary of State for Western Hemisphere Affairs.
Jacobsen is due to lead a U.S. delegation to Havana in January to begin normalizing diplomatic relations with Cuba that had been severed since 1961.
But domestic opponents of Obama's opening to Cuba have vowed to stop it, saying Cuba does not deserve a softer U.S. policy as long as it continues to control the media and repress political opponents under a one-party system.
"The Castro regime's latest acts of repression against political dissidents in Cuba make a mockery of President Obama's new U.S.-Cuba policy," said U.S. Senator Marco Rubio, a Cuban-American Republican from Florida and a potential 2016 presidential candidate.
Cuban President Raul Castro praised Obama for changing U.S. policy on Cuba but also warned that opponents to Cuba's government may try to sabotage better relations.
Yoani Sanchez said on Twitter that police detained her husband Reinaldo Escobar and dissident leader Eliecer Avila outside her home in Havana, taking them away handcuffed in a patrol car.
Escobar is editor-in-chief of the dissident news and opinion website 14ymedio.com and Avila is the leader of the opposition group Somos Mas (We Are More).
Reached at her home, Sanchez declined to comment.
A Reuters reporter saw a police car with two officers and an Interior Ministry official parked outside her apartment building.
Upon announcing his new Cuba policy, Obama said Cubans should not face harassment or arrest for expressing their views and that his government would continue to monitor human rights.
The deal on renewing ties included a prisoner swap in which the United States freed three Cuban spies and Cuba agreed to release U.S. aid contractor Alan Gross, a Cuban who spied for Washington, and 53 people who the United States considers political prisoners.
So far, the 53 have not been identified and dissident groups say none of their activists has been released since the Dec. 17 announcement. (Reporting by Daniel Trotta, Daniel Wallis, Nelson Acosta, Rosa Tania Valdés and Enrique de la Osa in Havana, and Francisco Alvarado in Miami; Writing by Daniel Trotta. Editing by David Adams and Ken Wills)