HAVANA, Cuba, Jan. 15--Rebel leader Fidel Castro warned the United States sharply today against meddling with Cuba's revolutionary justice. He declared that if U.S. Marines were sent to this island nation "200,000 gringos will die."

Gringo is a term used by some Latin Americans, often in a derogatory sense, for U.S. citizens. About 12,000 live in Cuba.

His threat presumably was prompted by statements of some U.S. congressmen that the State Department should act to halt the executions which have followed the fall of President Fulgencio Batista's dictatorship.

The reported toll of the firing squads rose Thursday to 195. "We are trying those who killed 20, 30 or 40 persons," Castro said," and they ask us to be humane!"

As to the executions, the State Department held to a policy of nonintervention. Asst. Secretary of State Roy Rubottom, who is in charge of Latin-American affairs, declared, "We are not going to intervene in what is essentially Cuba's affair."

Russians Want Conference

UNITED NATIONS, N.Y., Jan. 15--Anastas I. Mikoyan, Soviet first deputy premier, said today his country still wants a summit conference on world problems and the West cannot sidestep one forever.

Mikoyan, who will see Secretary of State John Foster Dulles a second time in Washington Friday, expressed interest in Dulles' statement that free elections were not the only way to reunify Germany.

But he complained that Dulles "did not come up with a constructive alternative" to the Soviet plan for confederation of East and West Germany by mutual agreement.

Washington speculation had been that Dulles' statement, at a news conference Tuesday, was a hint at a shift in the U.S. line aimed to provoke Mikoyan's interest and a change in the Soviet line.