Three men convicted of hijacking a passenger ferry last week were executed by firing squad Friday, a swift response by Fidel Castro's government to a recent string of hijackings to the United States.

The executions coincided with a crackdown and stiff prison sentences for scores of the government's most vocal critics -- both cases reflecting a new determination by Cuba to squash perceived threats to its socialist system.

A court sentenced the men Tuesday after finding them guilty of "very grave acts of terrorism," said a statement read on state television.

The three cases were appealed to both Cuba's Supreme Tribunal and the governing Council of State and all the sentences were upheld. "At dawn today the sanctions were applied," the statement said.

Cuban officials say that the four recent successful and attempted hijackings were caused in part on what it calls a lax attitude by American authorities toward hijackers who reach U.S. shores. They also say that they were part of a plot by the South Florida exile community to rupture migration accords between the two countries.

In Washington, the State Department condemned the hijackings but expressed concern that the cases may have been handled in a summary fashion. A statement said that such proceedings "are the hallmark of a totalitarian dictatorship like Cuba."

Cuba's Roman Catholic bishops issued a statement questioning both the executions and the political crackdown.

"Violence is not eliminated with more violence," it said, adding that they were also concerned about "long prison sentences imposed on political opponents."

Even as the sentences were carried out, Cuba announced the arrests of four men Friday who allegedly were planning to hijack a plane on Cuba's small Isle of Youth, south of the main island. The men were arrested late Thursday. The government said that they never boarded the plane.

Despite Cuba's claims, state television here last week broadcast a message by U.S. Interests Sections Chief James Cason asking Cubans not to hijack any more planes or boats. Cason warned that hijackers who reached American shores would be arrested, tried and kicked out of the United States after serving long sentences.

The government has accused Cason and the United States of fomenting dissent among government opponents, allegations U.S. officials have denied.

In less than a month, the government arrested, tried and convicted 75 dissidents. All were sentenced this week to terms ranging from 6 to 28 years on charges of receiving money from and collaborating with U.S. diplomats to undermine Cuba's leadership.

The statement identified those executed as leaders of the hijacking: Lorenzo Enrique Copello Castillo, Barbaro Leodan Sevilla Garcia and Jorge Luis Martinez Isaac.

"They shot him," cried the mother of Copello Castillo. "I love my nation. But now I no longer love Fidel," said Ramona Copello Castillo.

She said her son should have been prosecuted "but not by shooting him to death."

Four four other men received life sentences, while four others received sentences ranging from 30 years to two years.

International rights groups and representatives of the Cuban exile community were outraged.

"To execute these men is itself a human rights violation, and to do it less than two weeks after their alleged crimes shows a flagrant disregard of the right to a defense," said Jose Miguel Vivanco, executive director of the Americas Division of Human Rights Watch.

"Castro does this so you get outraged, so you overreact. What he's doing is cutting off Cuba's future," said Joe Garcia, executive director of the Cuban American National Foundation -- the nation's largest Cuban exile group. "

Capital punishment in Cuba is always carried out by firing squad but has been used sparingly in recent years.

No one was hurt in the hijacking of the ferry Baragua. A group of men armed with at least one pistol and several knives seized the ferry in Havana Bay early on April 2 and set sail for the United States about 50 hostages on board.

Later that day, the ferry ran out of fuel in the high seas of the Florida Straits and officers on the two Cuban Coast Guard patrol boats that chased them there tried to persuade the hijackers to return to the island.

The hijackers allegedly threatened to throw passengers overboard but eventually agreed to let the ferry be towed the 30 miles back to Cuba's Mariel port for refueling.

After the boat was docked in Mariel, west of Havana, Cuban authorities eventually gained control of the ferry and the suspects arrested and hostages freed uninjured.

The ferry was hijacked a day after a man who said he had two grenades forced a Cuban passenger plane to fly to Key West, Fla. The grenades turned out to be fake.