At least six prominent opposition political and labor leaders were released from jail here yesterday following a government announcement that it would free an unspecified number of political prisoners, apparently to meet one of three opposition demands to clear the way for negotiating an end to the current crisis.

The Broad Opposition Front has said that it will not enter into negotiations with President Anastasio Somoza until an estimated 350 prisoners arrested over the past month are released, and until martial law and censorhship decrees are lifted.

Two opposition leaders are known to have died in prison over the past month. One of those released yesterday, Adolfo Calero, a Broad Opposition Front official and Nicaraguan Coca-Cola Co. manager, said yesterday in the presence of the Managua Central Jail police chief that the six "wish to express our thanks for having been treated in a very humane way."

Meanwhile, U.S. special emissary William Jorden left Nicaragua after a round of meetings during which Somoza agreed to negotiate with the opposition through the mediation of a group of Latin American governments.

One foreign diplomat close to the discussions said Somoza still disagreed with Jorden over which countries would conduct the mediation. While Somoza pushed for a predominance of military-ruled governments for the mediating team - which will consist of four or five countries including the United States - the diplomat said Jorden had insisted that democratic governments must be in the majority.