MANAGUA, Nicaragua (AP) _ Dozens of youths yelling ''People power 3/8'' hurled rocks Friday into the headquarters of the main opposition group, where 200 people were discussing amnesty for political prisoners.

Later, Bayardo Arce, deputy coordinator of the ruling Sandinista National Liberation Front, told a rally of thousands of government supporters the leftist movement will not surrender power. Nor, he said, will it allow approval of reforms to the constitution which opposition groups are demanding to give them a greater voice in the government.

The Sandinista government called repeatedly in radio broadcasts throughout the day for Nicaraguans to attend the pro-government rally Friday night. Banners strung around Managua said, ''Reagan Will Be Defeated, Long Live the Armed People'' and ''We Are a Poor People, But Dignified.''

Friday was the anniversary of a 1967 demonstration against the Somoza family, who ruled Nicaragua for more than four decades. An estimated 200 people were killed when national guardsmen broke up the protest 21 years ago.

President Anastasio Somoza was overthrown by the leftist Sandinistas in 1979.

About 30 youths, most in their late teens, attacked the headquarters of the Nicaraguan Democratic Coordinate while members were meeting inside with families of political prisoners, the organization's representatives said. The mob grew to about 60.

Ramiro Gurdian, a vice president of the Coordinate, said several people sustained minor injuries from stones and flying glass.

Some of the youths yelled ''Somocistas 3/8'' People inside yelled ''Sandinista murderers 3/8'' and ''Liberty, liberty 3/8'' and threw the rocks back at the youths.

The youths ran away when about 15 police arrived, but they later regrouped. No one was arrested, witnesses said.

''You see how liberty in Nicaragua works? Lift the state of emergency and the turbas come,'' Gurdian said, using the term for Sandinista mobs.

Gurdian spoke to journalists outside the headquarters as a group of youths jeered and shouted ''Turbas are the people 3/8''

Turbas, with the support of the government, frequently harassed opposition groups in the early years of Sandinista rule. They have been largely inactive in the past few years as opposition activity has been limited by a state of emergency.

President Daniel Ortega said last week that several thousand prisoners could be freed if the United States or another country outside Central America agreed to take them. Otherwise, they will be granted amnesty after a cease- fire with the U.S.-backed Contra rebels is arranged, he said.

The amnesty and lifting of the 6-year-old state of emergency are among moves Nicaragua is required to take to comply with a peace plan signed by five Central American countries Aug. 7.

The Democratic Coordinate is made up of 14 anti-government political parties, unions and trade groups.

In the past week 12 of its members were detained by state security agents for questioning about a meeting with Contra leaders in Guatemala.

In a letter published Friday in the opposition newspaper La Prensa, one of the 12 said the chief of state security threatened him and his family with death for organizing the meeting.

Gilberto Cuadra, representative of the Superior Council of Private Enterprise in the Coordinate, said that Lenin Cerna ''came to the cell where I was and in strong words told me to stand up and immediately held me responsible for the meeting in Guatemala.''

''He said myself and Ramiro Gurdian promoted the meeting and that even though they were going to release us they would watch every step of my activities and that meant my death,'' he continued.

Cerna was unavailable for comment Friday.

The opposition leaders were detained for questioning about what Sandinista officials said was a CIA plan ''to cause conspiratorial and terrorist actions'' aimed at Nicaragua.

All 12 were released within a day of their capture.

At the rally Friday night, Arce declared, ''We will not allow ourselves to be attacked militarily from outside.''

''Reforms to the constitution will not be allowed,'' he said, adding that the Sandinista leadership will instruct its deputies in the National Assembly, where they now hold a majority, to block such moves.

''If the opposition takes to the streets, the Sandinista Front will go out into the streets too,'' he added.