Nicaraguan President Anastasio Somoza said yesterday that massive street demonstrations and unified opposition to his regime could convince him to resign.

If he did, Somoza said on Face the Nation (CBS, WDVM), it would be done in an "orderly" manner, with "an election and a constitutional assembly."

Somoza's expression of willingness even to entertain the idea of stepping down-an action he says the United States has been pressuring him to take-varied from many of his past comments.

Previously, he has indicated that his country's constitution effectively prevented his resignation.

Somza was interviewed in Miami on what he said as the final day of a U.S. vacation before he returns to Managua.

Nicaragua is embroiled in a bloody civil war, fought primarily with Sandinista Liberation Front guerrillas. It has been prolonged, in the view U.S. officials, by Somoza's unwillingness to relinquish any power despite strong opposition from numberous sectors of Nicaraguan society.

"If all concerned got together" in opposition to his regime, "I have to go . . . I don't need to be president," Somoza said.

He blamed outside influence for many of his country's problems and U.S. pressure for others. "Our friends, the United States, are not looking after their friends, the people of Nicaragua," Somoza said.

He said the U.S. cutoff of arms shipments in January 1976 served as (asignal to give these people [rebels] the idea that they could rise and do all of those things. They will say, 'This time the Somozas are going to go.' But they forget the Somozas have a party that has grass roots. They have an army that is loyal to the constitution." CAPTION: Picture, ANASTASIO SOMOZA . . . varies from past comments