President Carter rode into Belgrade today behind a yellow flatbed truck carrying about 25 American journalists and one nearly hysterical White House advance man pounding the truck's roof and shouting obscenities at the driver.

It was classic confrontation between an Eastern European security bureaucracy and an American presidential entourage accustomed to getting its own way.

The chaotic entry into Belgrade began normally enough this morning as Carter's black limousine, near the front of a motorcade that stretched for blocks, crossed the Sava River and swung a main thoroughfare of the capital city.

The street was lined with thousands of Yugoslavs who applauded politely and seemed genuinely pleased to see the president. Soon the removable roof of the presidential limousine was taken off and out popped Carter and Cvijetin Mijatovic, who is currently serving a one-year term as Yugoslavia's president under a new system that rotates the post among officials annually.

There could hardly have been a greater contrast in East-West politcal styles. The dour Mijatovic stood stiffly in the car, neither smiling nor waving to his countrymen. Carter, a broad grin on his face, waved constantly to the crowd as he passed.

And that is where the flatbed truck, loaded with reporters, photographers and television cameramen, came into play. When the president is in public, it is the American custom that he be photographed and filmed. When the scene is coloful, the crowds friendly and time an election year, the White House is especially anxious to see that things go smoothly.

But things did not go smoothly today for part-time White House advance man Richard Moore, who was in charge of the truck at the head of the motorcade.

The truck moved out to fast, pulling the cameramen out of range. Orders to slow were relayed to the driver through an interpreter and ignored. Moore began pounding on the roof of the driver's cab demanding that the truck slow down, but to no avail.

At a major intersection, Carter's limousine stopped and the president got out, plunging into the crowd to shake hands. Still the photo truck moved on, with the desperate White House advance man now standing on the running board by the driver's window shouting, "Stop this truck, you son of a bitch, or I will personally rip your lungs out."

The motorcade resumed moving after a few minutes, its first "photo opportunity" of the day already blown. Knowing another stop was planned, Moore then resorted to the ultimate capitalist threat, shouting to the interpreter: "You tell him if he doesn't stop the next time, he's not going to get paid."

"No bucks," added Randy Lewis of the White House press office in his own succinct interpretation.

By the time it was over, the truck had been allowed close enough to Carter's limousine for the cameras to catch the scene, but few if any pictures were taken at the president's two stops.

Moore and Lewis also learned that the culprit was not the driver or the Yugoslav security agent riding with him, but a higher ranking Yugoslav official who issued his orders to the agent by radio.

Informed of this, Lewis promptly made a peace offering, suggesting he buy the security man a beer and forget the whole unfortunate incident. The agent quickly accepted, stipulating that the beer be American. But he added: "Not yet. We'll still fight this afternoon and tomorrow."