Cuban exile leaders and President Fidel Castro resumed talks yesterday on the release of thousands of political prisoners and a new policy that would allow Cubans to join their families abroad.

Before the session, Miami banker Bernardo Benes, leader of the delegation of some 70 exiles invited here, predicted a major announcement by Castro soon.

"I think that before this is over, we will see a loosening of the locks on all the world's political prisoners," Benes said. "Something major is going to happen."

Cuba says it jails now hold between 300 and 3,500 political prisoners. Castro has been quoted as saying a new policy could affect "millions" of Cubans - prisoners and former prisoners and their families, as well as others who wish to join relatives in exile.

Benes and Cuban government spokesman described the first day's talks - five hours on Monday - as cordial and constructive. But, Benes said, a large number of points remained on the agenda for yesterday's meeting.

The talks are being held in a small auditorium in the Palace of the Revolution, government headquarters, with Castro presiding from a chair at the center of a table at the front of the hall.

The talks are based on an agenda drawn up by Benes and his associates, covering a variety of types of persons who might be affected by the releases Castro has indicated he intends to carry out.

The government statement after the first day's session quoted Castro as saying that Cuba is moving on its own - not because of any international pressures - and that the matter is strictly one of Cubans dealing with Cubans.

Benes said he and Castro, on the basis of previous contacts, had agreed that some 3,000 prisoners could be freed and another 8,000 former prisoners might be able to leave with their families - some 20,000 additional persons.

Many thousands more would be affected by a family reunification policy. The exiles' agenda lists a series of classes of persons who would be allowed to leave and proposes terms for a quick and easy departure from Cuba.

Benes and others, including former prisoners who want to leave, continued to urge the U.S. government to expedite procedures for screening and processing applications of those cleared to leave.

A Department of Justice team is scheduled to depart Cuba today after screening some applications from Cubans already authorized to leave. Cuba has submitted lists of 288 prisoners and 818 family members who are to be allowed to leave.

The optimism of Benes and others in the delegation has created an air of high anticipation and mild euphoria around the Riviera Hotel, where scores of prisoners' relatives and former prisoners show up nightly to await news.

With his usual flair for the unusual. Castro himself whetted appetites by authorizing the release of 14 more prisoners over the weekend and two more after Monday's opening talks.

The prisoners authorized for released Monday are relatives of two members of the exile delegation. One of the prisoners, Alfredo Izaguirre, a former newspaperman in jail since 1961, was married Sunday to another prisoner's daughter.

Benes described Castro's action, which came after he met Izaguirre's mother, a delegate who returned for the first time in 17 years, as "one of the greatest wedding gifts ever made."