By DAVID GONZALEZ
 
MIAMI, April 11 — After a swift and secret trial, the Cuban authorities executed at dawn today three men who hijacked a ferry last week in a failed effort to reach the United States, a Cuban government statement said.

Four other men who used a gun and knives to help commandeer the boat were given life sentences, the statement said.

Although the death penalty has been infrequently applied in Cuba, the three faced a firing squad after having been convicted of what the statement called a "very grave acts of terrorism."

The executions came at the end of a week in which some 78 nonviolent dissidents received prison terms ranging up to 28 years for conspiring with American diplomats against the government.

The Cuban authorities, who said there had been seven hijackings in the last seven months, have long been critical of American policies that they say encourage illegal immigration. Human rights advocates, diplomats and American lawmakers condemned the executions as excessively harsh and questioned the motivation of the Cuban government.

"This is excessive in the extreme and raises the question as to whether or not this is altogether rational," said Wayne S. Smith, once the top United States diplomat in Havana. "It just goes too far. There is no rational, sensible explanation."

Today's executions were believed to be the first in two years, said Curt Goering, senior deputy executive director of Amnesty International U.S.A. He said in a statement that the men had "almost certainly" been denied due process.

"These executions, combined with the harsh sentences handed down last week to a number of recently arrested dissidents, is contributing to a climate of repression that hasn't existed in Cuba since the 1960's," Mr. Goering said.

The sister of Jorge Luis Martínez Isaac, one of the executed hijackers, said their mother had no idea he would face the firing squad today.

"The news they gave her today was, `Your son is dead,' " the sister, Yordanis Montoya, said in Miami, where she lives.

"When she learned he had been shot, she raced to the cemetery," she said. "When she got there, she said, `Down with Fidel,' and the police chased her away."

The executions came a day after the Cuban authorities said they had foiled a hijacking attempt on the Isle of Youth. According to Granma, the Communist Party newspaper, several men who assaulted a soldier and stole his assault rifle were arrested last night near the airport.

The hijackings have angered Cuban officials, who feel that American policy not only encourages illegal immigration but also is lenient on those who come here by force. United States officials this week repeated that American courts would prosecute anyone involved in a hijacking.

The Cuban authorities have been indignant, however, over the release on bail of several men who hijacked a plane to Miami last month.

Some political analysts had wondered if the authorities had sought the harsh sentences to deter any further hijackings, which could scare off already jittery tourists. Caribbean tourism in general has been down since the outbreak of the Iraq war, and Cuba's economy is heavily dependent on foreign visitors.

"They are executing people who hijack a boat," Mr. Smith said. "Will that reassure Americans who think it is safe to go to Cuba?"

The Cuban authorities see the hijackings as related to a series of provocations by the Bush administration and its conservative allies in Miami, according to an official statement in Granma. There is little doubt, Cuba analysts said, that the executions will only harden the government's critics.

The death sentences also infuriated those who, like Rep. Charles B. Rangel, Democrat of New York, had been advocating a dialogue with Cuba. "This about ends that discussion," Mr. Rangel said in a telephone interview. "I don't know how far they are going to go, but they know how to support their enemies and get rid of their friends."