The U.S.-led invasion of Grenada has intensified fears among the revolutionary leaders of Nicaragua and the guerrillas of El Salvador that they may be the next victims of U.S. military intervention in the Americas, according to officials of both groups and diplomats.

There also is a feeling among some diplomats, however, that the tough Grenadan and Cuban resistance on the Caribbean island may make the Reagan administration think twice before moving against a stronger foe elsewhere in the region.

Nicaragua is worried about the recent revival of a Central American Defense Council that could ask for U.S. action against Managua, just as six eastern Caribbean nations requested it against Grenada.

"Is the immediate invasion of El Salvador and Nicaragua already on the way?" Nicaragua's Council of State, or legislature, asked rhetorically in its statement on the Grenadan invasion.

Sunday, Commander Victor Tirado, one of nine directors of the ruling Sandinista National Liberation Front, warned in a speech that the United States may be planning to use military forces from Guatemala, El Salvador and Honduras to "spearhead" an invasion of Nicaragua, United Press International reported.

The Salvadoran rebels, who have offices here, said they were convinced that the United States would launch air strikes against their bases in northern El Salvador if they appeared to be defeating the Army of the U.S.-backed government there. They called the Grenada invasion "another step, the most serious of all so far," in the U.S. administration's "escalating intervention" in the region.

The Nicaraguan government and El Salvador's guerrillas--the Reagan administration's two biggest headaches in Central America--frequently have charged that Washington was planning to send Marines to quash their revolutions.

These accusations have formed part of a propaganda effort aimed at stirring up nationalist opposition to the United States.

Nicaragua's leaders "sincerely do believe that the history of their relations points toward intervention," a diplomat here said, adding that the Grenada invasion had compounded this fear. "Up to now they've been talking history. Now they've got one right now, and in the neighborhood."

Paradoxically, however, several diplomats said that the unexpected stiff defense in Grenada to what some see as Washington's "dress rehearsal" might discourage the United States from marching into other countries.

U.S. Marines would face much stronger resistance in Nicaragua--with nearly 30 times the population of Grenada--than on the Caribbean island, according to Nicaraguan and foreign military analysts. The Nicaraguan government has been passing out high-quality automatic rifles to thousands of militiamen and militiawomen, in part because of the perceived danger of a U.S. intervention.

"If the gringos come, we'll need every man with a weapon in his hand," Manuel Salvatierra, the Army's commander in chief in northern Nicaragua, said.

Likewise, the Salvadoran rebels already have several years' combat experience and bases in mountainous areas of northern El Salvador that are ideal for defense by guerrillas. Like the Nicaraguans, Grenadans and Cubans, the rebels are motivated by a militant ideology that apparently contributes to high troop morale.

The Reagan administration has said repeatedly that it has no plans to send combat troops to El Salvador or Nicaragua, although the president declined to rule out their use in El Salvador. So far the administration has limited itself to funding Nicaraguan antigovernment guerrillas, sending thousands of combat troops to Honduras for long-term maneuvers and supporting the Salvadoran Army.

There are at least two plausible developments, however, that could lead to direct U.S. military action in Central America, diplomats and other observers said. One is a war between Nicaragua and one of its neighbors, Honduras or Costa Rica, and the other is the prospect of a Salvadoran guerrilla victory.

Counterrevolutionary guerrilla activity already has led to cross-border shooting incidents between Nicaragua and its neighbors, both of whom have said that they would request U.S. military support in case of Nicaraguan attack.

The United States has promoted revival of a Central American defense group that appears aimed at creating a united anti-Nicaragua military front in the region. On Oct. 2, Honduras, Guatemala and El Salvador participated in reviving the Central American Defense Council, or Condeca, inactive since 1969. U.S. Gen. Paul Gorman, commander in chief of the U.S. Southern Command in Panama, attended the meeting.

Condeca could play the same role against Nicaragua as the eastern Caribbean states that called for U.S. action against Grenada. Counterrevolutionary guerrilla attacks in Nicaragua also could create an atmosphere of chaos that could lead Washington to defend intervention as necessary to restore order.

Private U.S. citizens here, many of whom are strong supporters of the Nicaraguan revolution, rallied outside the U.S. Embassy Wednesday to warn that they did not feel threatened living in Nicaragua and that Washington should not plan to cite their safety in justifying any intervention here.