Twenty-four demonstrators opposing U.S. policies in Central America were arrested this morning after they sat down at the front entrance of Fort McNair, blocking access to the military installation in southwest Washington.

The protesters were charged with "incommoding," or blocking a public way, a police spokesman said.

Fort McNair is the site of the Inter-American Defense College, which gives advanced courses to military and civilian officials from countries in North and South America. The college is funded by the Organization of American States.

About 60 protesters marched for an hour on the sidewalk opposite the fort chanting, "Block the troops, stop the war," and carrying posters that said, "Quarantine is an act of war" and "Stop Reagan's illegal secret war in Nicaragua."

At 9 a.m., three groups of eight people each successively crossed the street and blocked the entrance. After being warned to move by police, the protesters were arrested. The demonstration ended 40 minutes later.

"What we have accomplished is a successful, peaceful, and, we think, highly effective civil disobedience," said Barbara Gottlieb, a spokeswoman for the local chapter of the Committee in Solidarity with the People of El Salvador (CISPES), which helped organized the protest.

"The Reagan administration is creating a new Vietnam in Central America. The escalation in the last few months has been terrifying and totally dizzying," Gottlieb said. "American opposition to U.S. policies in Central America is really beginning to take hold."

"We targeted Fort McNair because it is part of the military, which is part of the decision-making in the Reagan administration," Gottlieb said.

"We stopped business at Fort McNair for one hour," a speaker for the protesters said as the demonstration ended.

But Lt. Col. Jamie Walton, press officer for the military district of Washington, said the protest "didn't cause any trouble. It was just an average work day." Traffic into the fort was diverted to one of the two other gates, he said.

Other groups participating in the demonstration include the Religious Task Force, the Sisters of Mercy, the Washington Committee Against Registration and the Draft, the Progressive Student Network and the U.S. Anti-Imperialist League.