Attempts to suspend a U.S.-inspired land reform program in El Salvador provoked an angry reaction yesterday in the Senate, where influential members threatened to retaliate by cutting off military aid to the Salvadoran government.

"If the Salvador government is reneging on the land-reform program, then it is the expressed opinion of this senator that, under the law, not one cent of funds shall go to the government of El Salvador," said Sen. Charles H. Percy (R-Ill.), chairman of the Foreign Relations Committee, in response to the action taken Tuesday night by El Salvador's rightist-dominated Constituent Assembly.

Percy's statement, and similar remarks by other senators, came despite efforts by the State Department to explain that the meaning of the assembly vote was unclear. In addition, reports from San Salvador indicated that the assembly action almost certainly will be vetoed by Salvadoran President Alvaro Magana.

In fact, as of last night, it appeared that the storm of anger on Capitol Hill would blow over once the full facts of the complicated situation became clear. But the incident illustrated anew that there is still strong latent resistance in Congress to President Reagan's controversial Central America policy and his administration's attempts to work with the rightists who captured control of the assembly in El Salvador's recent elections.

The land-reform law, part of which literally was written in the State Department and virtually imposed on El Salvador by U.S. pressure in 1980, ironically is regarded by almost all factions in that country as deficient in many respects and in need of drastic overhaul.

However, the Reagan administration seized on it as a symbol of the reforms it seeks to promote in El Salvador. In the face of Salvadoran complaints, the State Department called the program "a remarkable success story," and, prior to the Constituent Assembly elections, the administration stressed that future decisions about U.S. aid would depend in part on continuation of a successful land-reform program.

Similarly, Congress specified that, as one of the conditions for giving military aid to El Salvador, Reagan must certify it is making "continued progress in implementing essential economic and political reforms, including the land-reform program. . . . " The president, who made such a certification early this year, must do so again next month.

That is why initial reports of the assembly's action Tuesday touched such a sensitive nerve here. Originally, Magana, concerned that landowners had no incentive to plant cotton or sugar cane because they expected to have their land expropriated, had suggested temporarily exempting land used for these crops from the program.

That idea had generally broad support, including that of the Salvadoran military and the Christian Democrats who form the moderate opposition in the assembly. But, in passing the law, the majority rightists attached several provisions that also would exempt lands used for cereals, corn and livestock.

The State Department's initial reaction yesterday was to say that there was "disagreement and confusion" about the effects of the law. It also repeated the administration's support for land reform, but its statement was so filled with technical qualifying language that it gave the initial impression that the administration might be backing away from its former unqualified support of the original law.

In the meantime, at a markup session of foreign aid legislation, Percy exploded in outrage; and Sen. Claiborne Pell (D-R.I.), the committee's ranking minority member, threatened to introduce an amendment cutting El Salvador out of the aid program.

That caused a deputy assistant secretary of state, Stephen W. Bosworth, to rush to the committee room to plead for caution until things became clearer. By last night, reports from San Salvador were saying that Magana, under pressure from the military and moderates, would veto the law in any event.

As a result, administration officials privately said they believed the storm would abate. But, as some privately admitted, the incident illustrates the problem of trying to foster reform in a country where the assembly is dominated by reluctant rightists.