A big-finned vintage car sped past state troopers wearing gas masks as dawn broke over the world's largest beef slaughter house this morning, and when the driver tried to enter the plant gate he became so rattled he turned directly into the path of an oncoming car.

A chorus of union pickets waved their fists and shouted at the wreckage from across the road. "Scab, scab! You deserve worse than that!" And, as an ambulance arrived, "Let him die!"

One hundred state troopers, about one fourth of Nebraska's state police force, stood arrayed in riot gear on one side of Nebraska Highway 35. Several dozen grim strikers stood on the other in the seventh week of a strike at the Iowa Beef Processors Inc. plant here across the river from Sioux City, Iowa.

Violence began Tuesday, when Iowa Beef reopened its struck plant with non-strikers and non-union workers hired, the union says, at $2 an hour below what the union members were getting.

The strike began six weeks ago, when Iowa Beef rejected the union's offer of a two-year wage freeze and no cost-of-living increase for three years. The company has asked for a four-year freeze on all increases.

The confrontation is a mean and dangerous one in the old style. It contrasts grimly to recent "cooperative" negotiations, such as those in the auto industry, in the so-called era of union concessions.

At its center is a boom company known as a tough competitor, whose 1981 net profit was $57.8 million and whose approach to handling slaughtered cattle carcasses has revolutionized the industry. The company says that keeping wages down is a matter of its survival.

"We don't claim poverty. Of course we're making a profit," said Charles Harness, company spokesman. "We have a profit margin of a little over 1 percent. . . . The average in American industry is 4 to 5 percent. The profits cited by the union are on sales of $5 billion to $6 billion."

The striking meatcutters, for their part, look and talk as tough as the work they do. They are fighting for a living wage, they say, but they are also struggling to maintain the union's diminished grip in the industry.

After two days of sporadic violence, in which two dozen strikers have been arrested, 29 people injured and 70 vehicles damaged, the state troopers have mustered a combination of tear gas and muscle to force the unionists away from the plant entrances. But some strikers said the trouble is far from over.

If the troopers drive them away from the plant they'll go find the "scabs' houses," one young striker threatened.

The trouble erupted Tuesday on the dawn shift, when the sprawling, snow-white plant reopened its operations at a diminished level using non-union workers and union members willing to cross the lines. It had shut down six weeks earlier, when 2,450 members of the United Food and Commercial Workers Local 222 walked off their job.

As the first workers drove in that morning 300 protesters pelted them in their cars with bricks and rocks.

Union attorney Harry Smith has charged that Iowa Beef deliberately provoked Tuesday's violence to "make the union the bad guys" in the public view. He cited as evidence such tactics as the company's allowing newspaper photographs of job applicants at the company's employment office.

Watching somebody take your job for $2 less is like "the husband who has to stand by and watch some other guy have relations with his wife," he said.

Local labor officials also fault the company for rejecting what they say was a chance to keep operating an extended contract while negotiations continued.

Still, outside the plant today, most strikers spoke regretfully of the violence, blaming it on "on a few hotheads." The man injured when he turned in front of an oncoming car turned out to have minor injuries.

But other strikers were more militant, although they refused to have their names attached to their tough talk. "The only way we can stop 'em is with violence," said one in a white T-shirt, a pack of cigarettes rolled up in his sleeve. "We'd like to stop those people and just talk to 'em, but they won't let us."