It was the kind of event that Argentina's populist Peronists ruefully have come to expect. Called here for a party congress, dozens of delegates arrived to find their entrance blocked by beefy guards even as unaccredited gangs of toughs pushed their way in.

Inside the smoky downtown theater, a small group of leaders began to ram through resolutions on du- "It's not going to be a struggle of two days. It's going to be a struggle of five years." -- Ruben Cardoso, Peronist congressman bious voice votes as their intrusive followers shouted down anyone who disagreed. A dissident provincial governor who arrived on the scene was assaulted and had to be escorted away by police.

Once again, Argentina's largest political movement appeared to succumb last month to the muscle politics that have at once held it together and slowly sapped its strength since the death of founder Juan Domingo Peron in 1974. This time, however, the leadership's tactics have backfired.

Hundreds of party delegates marched out of the hall that afternoon, leaving the nominal leadership to preside over a valueless party election. Within days, the dissidents announced plans for their own party congress. The Peronists' Justicialist Party is now formally split for the first time in its 40-year history, and after several weeks of failed negotiations, seems in danger of exploding into fragments.

"It's a pretty ugly situation," said Sen. Oraldo Britos, a leader of the convention walkout. "We are finally facing the crisis of the lack of valid leadership since Peron."

In a larger sense, the Peronist split also has initiated one of the most difficult and long-postponed challenges of Argentine democracy. In the decade since Peron's death, his followers repeatedly have failed to create a stable structure for their unique alliance of unions, the urban and rural poor and middle-class nationalists of both the left and right.

Nevertheless, the party and its labor unions have remained a powerful political force, with nearly 3 million registered members and 12 provincial governors. The result is that the Peronists' malaise and anarchic feuding have tended to affect the nation as a whole, impeding the consolidation of strong democratic institutions.

"What becomes of this movement is crucial to the country, not just us," said Humberto Romero, a Peronist leader from Corrientes Province. "The government needs an organized democratic opposition with a clear leadership. And the country needs a coherent political alternative that is not the military."

During the first year of Radical Party administration under President Raul Alfonsin, "there has been no 'Peronist' party line, no generally accepted set of party authorities, not even anything that could be described as a party organization," wrote political columnist James Neilson in the Buenos Aires Herald.

Consequently, Alfonsin has been spared from concerted opposition to his major policies, but at the same time has had to endure the scattershot attacks of myriad unions and Peronist fragments uncontrolled by nominal party authorities.

Some Radical leaders have begun to hope their center-left party can absorb the bulk of the undirected Peronists, creating a new mass movement that could dominate Argentine politics indefinitely. Other political analysts say the Peronists' troubles will more likely strengthen a series of small parties in this year's federal elections, multiplying the factions in an already fractious Congress.

It is the prospect of another painful election defeat that finally has precipitated the Peronists' internal crisis.

Despite its stunning loss of the 1983 presidential election, the movement has continued to be formally ruled by the coalition of labor leaders and provincial bosses led by veteran union strongman Lorenzo Miguel, who forcibly bound the party together last year.

Miguel and Buenos Aires provincial chief Herminio Iglesias are widely accused of alienating much of the Peronists' publc following. Meanwhile, titular party president Isabel Martinez de Peron, after a fitful attempt to establish authority last May, has remained in Spain and lapsed into her previous, eccentric silence, refusing to receive or even speak on the telephone with any party member.

Opposition to the leadership united a broad coalition of Peronists from interior provinces. Reformists and leftists who arrived at last month's party congress determined to oust both the old party leaders and the movement's authoritarian vestiges. "The idea was to have a real democratic debate not just about leadership, but about the future of the movement," said Britos.

But Miguel and Iglesias had other ambitions. With control over the security force, Iglesias prevented a number of dissident delegates from his own populous province from entering the convention hall, then arranged for hundreds of his raucous followers to slip inside.

"They took up strategic positions and started to take over the convention," said Britos, who was the congress' secretary. "They shouted down or acted violently against anyone against them. I pointed out a couple of times that there were no guarantees of fairness. The third time, I left."

The remaining delegates went on to elect Peron president and Iglesias secretary general of the party and gave Miguel a vice presidential slot. By then, however, the damage had been done. More than 400 of 640 delegates had left the convention, and the new leadership was repudiated by 10 of the 12 Peronist governors, 20 of its 21 senators, and 79 of its 111 congressmen.

The dissidents have scheduled their own convention for early next month in the interior city of Tucuman, and efforts at negotiations between the two sides so far have gone nowhere. "We are going to elect a new leadership and develop a program," said Britos, "whether or not the others come."

Even if the competing leaders avoid a final split in Peronism -- or the definitive collapse some analysts now predict -- the work of rebuilding the party barely will have begun. The reformist Peronist leaders concede that they are divided over issues ranging from procedures for choosing a new governing council to what status should be accorded former president Peron.

Moreover, the Peronists still have not begun to thrash out ideological issues, which divide the party even more sharply than the leadership question. "The center, right and left are in agreement on what we don't want, but we have not even begun to analyze what we do want -- and that is where the trouble is going to begin," said Ruben Cardoso, a leading congressman.

"It's not going to be a struggle of two days. It's going to be a struggle of five years. That's the only way out for Peronism."