President Luis Herrera Campins' strong support of El Salvador's besieged civilian-military government has come uner increasingly open criticism here, with senior Venezuelan opposition leaders charging his administration is pursuing an "interventionist" policy and covertly sending arms and military advisers to aid that country.

The charges, which have been denied by the government, also include an allegation that Venezuelan security officials are collaborating with the U.S. Central Intelligence Agency in establishing an espionage network in El Salvador.

State Department officials in Washington consistently have sought to portray Venezuelan involvement in El Salvador as at least substantial as their own, and have pointed to the "joint" policy of support for the Salvadoran government as evidence that the United States supported by other Latin American governments.

While denying sending military aid or advisors to El Salvador, Herrera Campins has been so firm in his public diplomatic and economic support that Venezuela is now seen as the United States' on only strong regional ally backing the Salvadoran government.

Many observers of Venezuelan politics believe Herrera Campins' strong backing stems at least in part from his desire to move as far as possible from the policies of his social democratic predecessor, former president Carlos Andres Perez. Perez, at times in vocal opposition to the United States, was one of the strongest supporters of the victorious Sandinistas during the 1979 Nicaraguan civil war.

"There have been officials of the National Guard and the Venezuelan Army used as advisers to the Salvadoran Army," Eloy Torres, vice president of the Venezuelan Chamber of Deputies and of the Movement for Socialism party, declared yesterday. He also said that individual Venezuelan Christian Democrats "fly constantly to El Salvador in private planes carrying arms and assistance."

Torres spoke at a meeting called after the arrest this week of a Venezuelan film maker working in El Salvador. He was charged with having ties to Salvadoran guerrillas. The meeting demanded his release. Venezuelan government officials said earlier the film maker, Nelson Arrieti, was delivering money to guerrillas.

Venezuela has been generous with El Salvador in its six-year-old program of loans to Central American countries struggling with increased oil prices -- a program that also includes the ruling Sandinistas in Nicaragua -- and just yesterday, Venezuelan Central Bank officials signed agreements for a renewed loan of $31 million to El Salvador.

Herrera Campins, a Christian Democrat who sees his country as the continental stronghold for an international Christian Democrat movement, looks upon the Salvadoran struggle as a fight between Christian Democracy and Marxism. Newly installed Salvadoran President Jose Napoleon Duarte, who now rules at the head of a previously-supreme junta, is a Christian Democrat who lived in Caracas for seven years. He had been exiled after winning presidential elections in 1972 -- by the same military with whom he now shares the government -- and is reported to have worked closely with Venezuelan party members.

"If a Marxist regime installs itself in El Salvador," said Aristides Calvani, an ex-foreign minister and Christian Democratic Party official who has been especially eloquent in his support of the Salvadoran government, "it will represent a profound unbalance in the area. It is most important for international Marxism to triumph in El Salvador, because it permits them to establish the beginning of a bridge. They intend to have a Marxist government in Nicaragua, too."

Calvani, reportedly working closely with U.S. officials here, in Washington and in El Salvador, has flown to the Salvadoran capital several times since Duarte was installed as president last month in what a number of informed observers have said was a Calvani-authored plan to lessen military influence in a last-ditch effort to save that government.

Calvani denied opposition charges that the trips were to give advice and aid to the Salvadorans and their security forces. He said the visits were part of his duties as secretary general of the Organization for American Christian Democracy, an international group of which Duarte is president. "They are not secret trips," Calvani said. He said the charges of Venezuelan military aid in El Salvador were "totally false."

Citing "reliable sources," Torres and the oppositions leaders joining him have accused Venezuela of working in conjunction with the CIA to create a Salvadoran spy network they say is headquartered at the San Salvador Sheraton Hotel. The network is alleged to include seven officials of the Venezuelan secret police and to go by the code name "Centaur."

Jose Vicente Rangel, the Movement for Socialism's unsuccessful presidential candidate, said there are several dozen additional Venezuelan police officers working under diplomatic cover in El Salvador, along with 10 military attaches. "The government seems to be implicated in a very grave interventionist policy," he said.

Both the United States and Venezuela have been defensive about charges that Venezuela is somehow doing the larger nation's bidding by replicating so precisely its policy of support for the Salvadoran junta. "Their policy is independent from ours," said a diplomatic source. "We coincide. But they don't want to be seen as our agents. They're in there with much more commitment then we have."