WASHINGTON (AP) _ The younger brother of Najib, the Soviet-backed leader of Afghanistan, denounced him Friday as ''mentally deranged'' and in a losing struggle to keep the country from falling to rebels armed with American Stinger missiles.

Appearing at a news conference at the State Department, Sediqullah Rahi, 37, and his wife, Soraya, described their escape and defection to the United States. Rahi said the escape was arranged by Ahmed Shah Massoud, leader of one of the factions at war with his brother's forces, and required them to change getaway cars seven times.

The couple and their two small sons were given refuge by the U.S. government immediately after their secret arrival Wednesday from West Germany. They went there from Pakistan after spending several months hiding in the Afghan mountains under Massoud's protection, the Rahis told reporters through an interpreter.

''We were constantly under surveillance and they were following us,'' Mrs. Rahi said. Her bearded husband nodded his concurrence.

Rahi worked as a banker for the pro-Soviet government until he broke with his brother in 1981. He said he was kidnapped by Afghan agents, taken to East Germany and then the Soviet Union and also was jailed in Afghanistan.

''I do have some political disputes and disagreements'' with Najib, he said. ''It is an infamous regime. He is not being supported by anyone so far as the Afghans are concerned. His only support is the Soviet Union.''

The Soviets are in the midst of withdrawing their troops from Afghanistan where they had backed the Kabul government in an 8 1/2 -year war with the rebels.

Once the Red Army is gone, Rahi said, the government will fall. ''He has tried to impose his regime by way of bayonets,'' Rahi said of his brother. The rebels ''are the true fighters and defenders of our country.''

Rahi said he expected the Soviets to try to rescue elements of the pro- Moscow government by arranging a coalition with other Afghan forces. He said Najib might be given haven in the Soviet Union or another communist country.

''He is mentally deranged and the sadist type,'' Rahi said. ''He always tries to present himself as a very strong and forceful person.''

Asked about the Afghan president's claims that he could survive without the support of Soviet troops, Rahi said: ''Sheer lies.''

There are two younger brothers. Tradition binds them to Najib even if they disagree with him, Rahi said.

He said the two brothers were trained in Cuba and are working for the KGB secret police.