The United States expressed alarm Thursday over the Cuba's widening arrests of pro-democracy dissidents, which the State Department described as the worst act of repression by the Fidel Castro government in the last decade.

The State Department is condemning the round-up of Cuban dissidents as "outrageous" and it is appealing to other countries to join the United States demanding their immediate release.

The crackdown by the Castro government began earlier this week, with authorities in Havana accusing dissidents of being part of an anti-government conspiracy led by the head of the U.S. diplomatic Interests Section in Cuba, James Cason.

The State Department bitterly complained about the arrests on Wednesday, calling them an appalling act of intimidation against those seeking democratic change on the communist ruled island.

Briefing reporters here Thursday, State Department spokesman Richard Boucher said the crackdown only worsened after the initial U.S. complaint. "At least 55 and perhaps as many as 70 Cubans have been arrested for seeking their fundamental rights. We believe that there are three times as many people detained today as there were at the same time yesterday. This is now the most egregious act of political repression in Cuba in the last decade," he said.

Mr. Boucher said those arrested include economist and former political prisoner Marta Beatriz Roque and two associates, detained, he said, as they conducted a peaceful fast in Havana to protest the arrests of other dissidents. He said among others taken into custody were members of the Christian Liberation Movement, which spearheaded the Varela Project, an internationally acclaimed petition drive for democratic reforms.

The Castro government has mounted an unusual campaign of personal attacks on Mr. Cason, who has traveled throughout Cuba meeting dissident figures since becoming chief of the U.S. diplomatic office last September.

Cuban authorities this week announced restrictions on the movement of U.S. diplomats, requiring specific permission for them to leave the Havana area, and prompting the State Department to take similar steps against Cuban officials posted in the United States.

Cuba has said it will put the dissidents on trial, raising fears they may be prosecuted under a law that provides prison terms of up to ten years for publishing allegedly "subversive" materials provided by the U.S. mission in Havana.