The official Soviet news agency Tass accused the United States yesterday of planning a sea blockade of Nicaragua that it said would amount to an act of piracy.

In a commentary attacking Washington's plans for joint maneuvers with Honduras, the agency said the U.S. Navy had already lined up vessels to intercept ships heading for Nicaraguan ports and prevent supplies from arriving.

The report appeared a day after crewmembers of the Soviet freighter Ulyanov reported on arrival in the Nicaraguan port of Corinto that they were halted 55 miles off the coast by a U.S. destroyer. A Navy spokesman said yesterday in Washington that the guided-missile destroyer Lynde McCormick was executing routine identification procedures in asking the freighter Saturday, by radio, for its name, destination and cargo. There was no attempt to stop, search or interfere with the freighter, he added.

In Managua, Soviet Secretary General of Foreign Affairs Yuri Fokine told a news conference concluding his visit that the United States was "flexing its muscles" to "coerce Nicaragua." But when asked if the Soviets would provide military aid, he replied: "We will support Nicaragua politically in all forms."

The Tass report said, "The Reagan administration intends to practice downright sea piracy, similar to that of the pirates and buccaneers of the past. It added, "The Reagan administration is playing with fire."

President Reagan said at a news conference last week that the Ulyanov was carrying arms to Nicaragua. Defense officials have said that Navy ships off Nicaragua will practice quarantine maneuvers. But the Navy spokesman said yesterday that the hailing of the Ulyanov was not part of any quarantine action.

Asked if it were not unusual for a Navy ship to inquire about cargo, going beyond asking flag and destination of a passing vessel, the spokesman said, "a little bit." But he said the skipper or officer of the deck on the McCormick was acting within longstanding guidelines.

When hailed by the McCormick about 55 miles off the Nicaraguan coast, the Ulyanov identified itself and said it was carrying "general" cargo, according to a Navy spokesman. Reuter news agency quoted port officials in Corinto as saying that the unloading of the Ulyanov had been postponed until Friday because of congestion in the port.

In another development, Nicaragua said it has freed 58 Miskito Indians accused of subversion and admitted that some officials abused members of the tribe in counterrevolutionary crackdowns.

Interior Minister Tomas Borge announced the release Tuesday and said that of 320 Miskitos still imprisoned, others will be freed soon.

The Reagan administration frequently has denounced what it claims is the mistreatment of Miskitos and other Nicaraguan Indians by the leftist Sandinista government.

The Sandinistas began resettling thousands of Miskito, Rama and Sumo Indians two years ago from their ancestral villages on the Caribbean coast to inland camps. Thousands of Indians fled to Honduras, where some have since joined other Nicaraguan rebels fighting to overthrow the Sandinista government.