Heavy fighting between Sandinista guerrillas and National Guard troops swept northern Nicaragua yesterday as President Anastasio Somoza began an eight-day Easter vacation in the United States.

Somoza's Sunday departure in a private plane followed an announcement by the country's Central Bank president that Nicaragua will devalue its currency by 43 percent.

The devaluation is part of a requested $40 million International Monetary Fund loan agreement that the government hopes will help relieve severe balance of payments and internal deficit problems stemming from continuing political turmoil.

Despite high yields in this year's coffee and cotton crops, the country's chief exports, Nicaragua's financial situation has grown progressively worse as investor and creditor confidence has plummeted since a civil war between the National Guard and guerilla-led civilians last September.

Announcing the devaluation, which will alter the exchange rate of the Nicaraguan cordoba from seven to 10 to the dollar, Central Bank President Roberto Incer blamed the country's problems in part on "United States political interference" in the IMF.

Last fall, Nicaragua withdrew its application for a $20 million IMF loan after the United States privately indicated it would oppose the loan by questioning the government's figures.

The current $40 million request is under a different loan procedure known as a standby credit agreement, which allows the IMF to impose strict fiscal austerity measures, such as devaluation, on the borrowing government.

Diplomatic and banking sources indicated that the agreement itself, arrived at during a recent three-week IMF investigative mission to Managua and subject to IMF board approval expected in late May, also includes provisions for increased taxation and cuts in public spending.

While fighting in Nicaragua has continued sporadically since the September civil war, it increased dramatically over the past weekend when the Sandinistas attacked the northern city of Esteli and a number of surrounding towns.

Reports in Managua's opposition press and denied by the government claimed that the guerrillas had shot down two small government warplanes on Sunday. The National Guard meanwhile said that at least 28 guerrillas had been killed in the battles.

Telephone lines to Esteli were cut yesterday, and there was little confirmed information, but radio reports indicated that the guerrillas were in control of parts of the city as the fighting continued.

News services reported that the Pan American highway, Central America's main road link that runs through Esteli, was closed as both the National Guard and the guerrillas set up roadblocks.

At the same time, fresh fighting began along Nicaragua's southern border with Costa Rica, where the government has intesified air patrols to prevent incursions from guerrilla camps in the neighboring country.

Speaking to wire service reporters in Topeka, Kan., where his son, Julio, attends school, Somoza said yesterday that "the country is under control" and that he had no confirmation of reports that Esteli had fallen to the rebels.

In answer to speculation that he has permanently fled Nicaragua in favor of a provisional government formed within his own political party, Somoza said "I am the president of Nicaragua and I didn't make any deals with anybody . . . I am coming over here to see my son, and that's it."

Informed sources said Somoza was expected to fly to Miami, by way of Houston, on Tuesday. They noted that Somoza suffered severe heart attack in the summer of 1977, when he was hospitalized in Miami, and said he has been attended by physicians in both cities.

The sources said Somoza had waived Secret Service protection normally given visiting heads of state, and had not requested any meetings with administration officials.

Somoza has been at odds with the U.S. government since he rejected a proposal for a nationwide referendum on his continued presidency last year.

In February, the Carter administration cancelled economic assistance to his government.

At a weekend meeting in Pittsburgh, some 600 academic specialists belonging to the Latin American Studies Association adopted a resolution sharply criticizing past policy toward Nicaragua and calling for withdrawal of the U.S. ambassador.

The resolution, passed by a voice vote, declared that the United States had maintained Somoza in power and demanded that Nicaraguans be allowed to determine their own destiny. CAPTION: Picture, Sandinista guerrilla leaders proclaimed unity at a March 7 clandestine meeting. Left to right: Jaime Wheelock, Daniel Ortega, Tomas Borges, Henry Ruiz, Victor Tirado, Humberto Ortega.