Just blocks from Army headquarters sits a massive office building whose shuttered windows and peeling white paint embody one side of the broadest and most intricate power struggle within military-ruled Argentina.

Huge neon letters on the old building's top floor read CGT--the Spanish abbreviation for General Confederation of Labor, once the largest labor organization in Latin America and the heart of the mass movement that transformed Argentina's modern history.

Now, the sign is dark and the vast building is empty except for a military colonel completing the task of "liquidation" of the old nationally syndicated unions ordered 2 1/2 years ago.

And yet, several miles away in an old walkup apartment, dozens of men in work pants and loose shirts gather every afternoon to plan meetings and job actions and hand out proclamations to a waiting press.

"This," insists Saul Ubaldini, a lean, former brewery worker who has been jailed six times in six years of military rule, "is the General Confederation of Labor--in fact if not in law."

Ubaldini's organization, formed a year ago by labor leaders once at the center of the CGT and the Peronist movement, in reality bears little resemblance to the sprawling apparatus that filled the downtown building, which the Army now operates.

But the reborn CGT and other nominally illegal organizations like it have come to represent one of the strongest challenges to the military "process of national reorganization" that the government of Gen. Leopoldo Galtieri has launched.

Despite years of arrests and repression and the reorganization scheme for the country's 1,100 local unions,resurgent labor leaders have reconstructed their organizations outside the armed forces' control.

In recent weeks they have begun to overcome their internal divisions and unify for the first time against the Galtieri government.

Galtieri, at the same time, has sped up his plans for "normalizing" unions under the military, that is in essence, depoliticizing them and putting them under strict government control in order to prevent the old Peronists from taking over.

At stake in the struggle is not just control of salaries, but a vast system of union-controlled social services and the root political organization of Peronism, the populist, nationalist, state-oriented movement that the military ousted from power in 1976.

As a labor minister and later as president for nine years in the 1940s and 1950s, Juan Domingo Peron transformed the new proletariat of a rapidly industrializing Argentina into a loyal personal following by encouraging the formation of large unions in almost every economic sector, then granting them unprecedented wages and control over huge social programs and properties that ranged from hospitals to hotels.

Even after Peron's death in 1974, all organized labor continued to be controlled by the CGT, which, in turn, was controlled by a solid bloc of union leaders who were also Peronist party directors.

For the past six years, the military has tried to uproot this network of labor and politics. Hundreds of Peronist labor leaders have been jailed, many have been barred from union office and a significant number have simply disappeared. All formal labor activity remains banned, and some 50 national union syndicates are under the control of appointed military officers.

Now, most of the government's violent tactics have given way to the elaborate, multistage "normalization" process.

"Everyone knows that the majority of the unions are led by Peronists," explained one high Labor Ministry officer in an interview. "We don't pretend that it's different, and we have to accept it. But what we want is that political views are channeled through a political party--and that labor does not become a political party."

Government officials now display elaborate charts showing the progression of this "depoliticization" of the unions--which would have the effect of depriving the Peronist party of half its formal organization--and maintain that the process is headed for a smooth completion by 1984, at the end of Galtieri's presidential term.

In reality, however, the military is widely perceived here to be losing its grip over the old Peronist labor structure. The reorganization scheme has been repudiated by most labor leaders and by the U.N. International Labor Organization, and most all of the country's major unions are stalled at its preliminary stages.

Personist leaders who were once in jail or in hiding are now working in new offices and openly reestablishing the joint labor-political directorship of the Peronist movement. In many cases this is done as a parallel structure to the official, legal union leadership.

CGT Secretary General Ubaldini, during a recent interview in his bare, new office, flung open the shutters. "You see those men down there?" he asked, pointing to four well-dressed men sitting by the open windows of a small cafe across the street. "Those are military agents disguised as civilians. They follow everything we do and watch every person who comes in the door.

"But they have failed totally to stop the CGT and to stop the Peronist movement in the unions," Ubaldini said. "They have taken away our legal status, but they haven't been able to stop the workers from supporting us."

The question of the workers' support is actually less clear-cut than Ubaldini and the other labor leaders maintain.

Under government control, the number of unionized workers has fallen from about 7 million six years ago to between 3.5 million and 4 million now, according to estimates by unionists.

Years of systematic pressure by the armed forces and suppression of attempted uprisings have greatly weakened the will--if not the sentiments--of rank-and-file workers. "There is a feeling of resignation among many people," said Alfredo Carrazo, a Newspaper Workers' Union leader who now manages a labor news bulletin.

But an even greater obstacle to effective labor opposition to the government has been the bitter disputes among the old union leadership, which has been sharply divided ever since it resurfaced over both the proper tactics for approaching the government and the future of the Peronist movement.

On one side are the hard-line leaders of the CGT, who believe in what Ubaldini calls "a permanent state of mobilization" against the government. "We want to unify the labor movement," Ubaldini said, "but we want unity for a program of action. We don't want to join other unions who don't want to proceed."

On the other side is another large group of union leaders, many of whom retain their formal offices. These men are also loyal Peronists but have maintained that the best course for labor and the party is to bargain with the government while rebuilding support within the country as a whole.

But since Galtieri took office six weeks ago, signs have emerged that the labor leaders are uniting on a tactic of open opposition to the government. The National Confederation of Labor (CNT), believed to be the largest labor confederation, has called for a "program of opposition" for the first time.

Until now it has publicly opposed illegal strikes in favor of negotiations with the government. But last week it began distribution of anti-government leaflets that led to the arrest of six persons.

"We understand that there is no more time," said Jorge Triaca who heads CNT. "We now only have one road--to begin confronting the military and force them to give back what they have taken."

Two weeks ago, a major breakthrough came for labor unity as feuding CNT and CGT leaders of the country's foremost activist union, the Metalworkers, united.

Union leaders of both sides now say this year will bring the real test of the struggle between the old Peronist leadership and the armed forces.