BY JUAN O. TAMAYO
jtamayo@herald.com

An angry Fidel Castro on Sunday threatened mass protests and a boycott of upcoming U.S.-Cuba migration talks unless Washington agrees to return child rafter Elian Gonzalez within 72 hours.

''If they are halfway intelligent, they will announce the return of this child before 72 hours, the Cuban president said in a speech broadcast early in the day. ''It's already difficult to restrain our people.

''I hope that Cuba's numerous friends in the United States and in other parts will begin to organize committees for the release of the child kidnapped by the United States. Heaven and earth will be moved.

Castro said he was infuriated by the way his enemies in Miami had embraced the custody case as their own, and criticized television images of the child surrounded by toys and wearing a T-shirt of his nemesis, the Cuban American National Foundation.

He promised a ''battle for world opinion'' to bring Elian home. The child turns 6 today.

Spencer Eig, the Gonzalez family attorney in Miami, said he is confident Cuba has no chance of getting the boy back just because Castro is making threats.

''As an American, I'm proud that this country doesn't respond to threats,'' Eig said. ''Whatever opinion people may have about this case, I'm confident everyone joins me in saying, 'Mr. Castro, leave this boy alone.' ''

In Washington, the State Department said it would not comment on press reports about Castro's threats.

The Cuban leader's outburst cast an ominous shadow over U.S.-Cuba migration accords negotiated in 1994 and 1995 to avert another crisis like the 1980 Mariel boatlift and the 1994 flight of about 36,000 rafters.

At best, the dispute could force the cancellation of talks set for Dec. 13 on progress in implementing the accords. At worst, Havana might stop accepting the repatriation of would-be refugees intercepted by the U.S. Coast Guard in the Florida Straits.

Cooperation on migration issues has become one of the pillars of U.S.-Cuban relations since the collapse of the Soviet Union ended Cuba's status as a threat to U.S. national security. Castro's comments signaled a significant escalation of Cuba's campaign for the return of Elian, rescued Thanksgiving Day from an inner tube after his mother and 10 others drowned when their boat capsized.

The Clinton administration has said it will allow Florida courts to rule on Elian, now living with relatives in Miami. But his father, Juan Miguel Gonzalez, 31, has demanded his return to Cuba.

CASTRO REACTS

Castro called the Washington decision a ''kidnapping and branded Florida judges ''mercenary and venal, corrupt to the very marrow of their bones.

Unless U.S. officials agree to return the child, ''they are going to see millions of people on the streets, he said in a speech welcoming back Cuba's delegation to the World Trade Organization conference in Seattle.

On Sunday, Cubans began demonstrating outside the U.S. diplomatic mission in Havana, the U.S. Interests Section, demanding the boy's return. The government has mobilized up to one million marchers for past protests there.

Several dozen Cuban soldiers were added Sunday to the three to four troops who normally guard the Interests Section on Havana's seashore Malecon boulevard, The Associated Press reported. The reason for the move was not clear. Calls requesting comment from the mission were referred to Washington.

EXPLICIT THREAT

Castro charged that Washington is not abiding by the 1994 and 1995 migration accords, but left it up to National Assembly President Ricardo Alarcon to make an explicit threat to boycott the Dec. 13 meeting.

''My recommendation would be that they make sure Elian is back before that date, because it is very hard to imagine that we could have any type of constructive discussion, Alarcon told a weekend rally in front of the home of Elian's father in the north-central city of Cardenas.

Alarcon was the lead Cuban negotiator of the migration accords and has participated in all the twice-yearly meetings designed to review progress in implementing them. They usually last one day, and alternate between Havana and New York or Washington.

Designed to discourage risky illegal departures from Cuba, the pacts require Washington to issue 20,000 visas a year to Cubans and Havana to accept the return of would-be refugees intercepted by the U.S. Coast Guard.

Cuba has taken back about 2,500 boat people intercepted since 1994, but hundreds more have managed to reach U.S. shores, winning U.S. residency after one year under the Cuban Adjustment Act.

SORE POINT

That law, approved by Congress in 1966 to favor refugees seen as victims of a communist regime, has long been a sore point with Castro, who argues that it encourages illegal departures of people merely seeking better economic opportunities in the United States.

Castro, welcoming back the World Trade Organization delegation, charged that the 1994 and 1995 agreements contain ''an express commitment to put an end to that Cuban Adjustment law. [But] they have done nothing.

U.S. officials have repeatedly denied that the 1994 and 1995 pacts contain such an agreement, directly or implied.

Castro has an interest in keeping the migration agreements in good working order, because they serve as a safety valve for discontent with his authoritarian regime and crippled economy.

But in the past, he has taken hard-line stands on even relatively minor disputes with Washington and at times appeared to intentionally detonate crises in order to torpedo improvements in U.S.-Cuba relations.

Castro told the WTO delegation that he would not attend the upcoming inauguration of Argentine President-elect Fernando de la Rua on Friday because the case of Elian Gonzalez ''is the issue that I must handle at this time.

Herald wire services contributed to this report.