The shadow of military rule has settled once again over Panama.

Although a civilian is still president, the increasingly visible role of Gen. Manuel Antonio Noriega and his Defense Forces in running the country has largely eclipsed last year's hopes for effective rule by an elected leader, according to Panamanian and diplomatic sources.

In place of those hopes, they say, has grown a mood of frustration and cynicism, encouraged by a widespread impression that President Eric Arturo Delvalle has proved willing to do what Noriega tells him.

"Delvalle has opted for the path of least resistance," said Jorge Ruben Rosas, a National Assembly member from the opposition National Republican Liberal Movement.

Delvalle, who was vice president, became president last September after Noriega forced the resignation of Nicolas Ardito Barletta, an economist who only 11 months earlier had become Panama's first elected president in 16 years. Panamanian offiicials emphasized then, as they do now, that the transfer of power followed the norms of Panama's constitution.

But, since then, the democratic institutions that, with U.S. prodding, had been allowed to begin functioning with the military in the background have come under the sway of more direct military power.

In a move viewed as a symbolic pilgrimage, for example, Delvalle walked through the streets of the capital last Oct. 11 from the presidential palace to Defense Forces headquarters to join a celebration marking the military coup that overthrew an elected president in 1968 and ushered in Gen. Omar Torrijos as the leader of Panama. In addition, Noriega was prominent at Delvalle's side when President-elect Oscar Arias of Costa Rica visited recently.

Strident demands for investigation into the decapitation last September of Noriega's longtime foe Hugo Spadafora, blamed on the military by Panamanian and diplomatic sources, have been ignored under Delvalle.

"We don't have a civilian government anymore," said Guillermo Cochez, another assembly member from the opposition Christian Democratic Party. "You should see the people in the National Assembly. They just take their orders directly from the military."

The Reagan administration has withheld about $40 million in badly needed U.S. balance of payments aid to demonstrate displeasure with Ardito Barletta's dismissal and the military's renewed visibility. The new U.S. ambassador here, Arthur Davis, also created a stir by declaring during his confirmation hearings in February that he would "keep pressuring the military about the civilian government."

But Mayin Corea, an assembly member from the promilitary Laborist Party, said most Panamanians believe Noriega enjoys support from the United States, particularly a Defense Department concerned about security for the Panama Canal and U.S. military bases here.

"He can do anything he wants if the Americans continue to support him," she added.

Rosas, dean of opposition legislators, said the military's role is unlikely to diminish without "a push from abroad," such as those given by the Reagan administration in Haiti and the Philippines, "which has not happened here."

Panamanian reliability is key to U.S. military policy in Central America. The canal, which reverts to Panamanian control in 14 years, has long been a major security concern. U.S. bases here also provide a home for the U.S. Southern Command, reconnaissance planes over Central America and military advisers rotating in and out of El Salvador and Honduras.

Noriega has cooperated in meeting these needs. In recent weeks, for example, he has allowed U.S. use of the main Panamanian military airfield while runways are repaired at Howard Air Force Base. Despite the aid cutoff, U.S. and Panamanian forces also went ahead with joint military maneuvers in January called "Kindle Liberty." Despite widespread discontent, Panamanian and diplomatic observers point out, there has been relatively little street violence directed against the government. A 10-day general strike last month provoked disorders that left one person killed by police and a number of shops damaged by bombs. But the economic measures that touched off the strike nevertheless were voted through the assembly after arm-twisting by the military, and the strike was called off.

Several foreign and Panamanian observers explained the return to calm as a reflection of the country's relative economic well-being compared with others in the region, in which antigoverment movements have turned to guerrilla war.

"Even though there is a lot of corruption, there is also a lot of money," one commented.

Many Panamanians remember the populist tone of military rule under Torrijos, he added, and Noriega frequently inaugurates public works projects and takes other steps to maintain that image.

A commentary in the promilitary newspaper La Estrella reminded readers Thursday that Panama must maintain "as a premise of its equilibrium and progress the permanent integration, solid, sincere and active, of its civilian resources with its Armed Forces.

"In Panama, this integration has come about through a phenomenon of spontaneous symbiosis, an understanding between the people in uniform and those who are not," it added.