SAIGON, Vietnam — Russian-built Vietnamese air force planes and helicopters zoomed overhead, ambassadors and bureaucrats cheered from the stands, and floats cruised down the avenue leading to the presidential palace through whose iron gates the first “North” Vietnamese tank had roared to victory 50 years ago.

The American ambassador to Vietnam, however, was conspicuously missing at the “Liberation Day” parade Wednesday marking the half century since the forces from Hanoi, the capital of the North, completed their conquest of the South more than two years after the last American combat unit had gone home. There was no official explanation, but the message was clear: Washington did not want to honor the regime that had dealt the Americans a bitter defeat.

“Vietnam is one, the Vietnamese people are one,” the general secretary of the ruling Communist Party of Vietnam, To Lam, orated before the parade began, but his message was more than mere rhetoric glorifying the army from the north. Rather, it was intended to inspire the dubious loyalty of Vietnamese in the south, once known as “South Vietnam,” while the leadership in Hanoi may be intimidated by the southerners’ booming economic success.

For the first time in any of these elaborate “Liberation Day” celebrations, the blue-and-red flag of the National Liberation Front, with a large gold star in the center, was on display almost as much as the solid red flag of the Communist regime, dominated by the same gold star. The NLF, of course, is the formal name for the Viet Cong, the southern guerrillas who were largely conquered by the Americans until “main force” North Vietnamese troops infiltrated the south, absorbing and replacing them.

“They want to inspire the loyalty of the south, to convey the sense that the southerners also contributed to final victory in the war,” a former editor of the National Catholic Reporter, Tom Fox, who lived and worked here for several years during the war, said. There are no signs of rebellion or even unrest here, but southerners never forget they exist under Communist rule while pursuing economic success. “They can do what they want as long as they don’t say anything against the party,” Mr. Fox said. The wealth of southern business, he believes, can seem overwhelming in Hanoi.

For Liberation Day, though, the atmosphere was all fun and games as thousands camped out on sidewalks all night hoping for a glimpse of the parade, many wearing Vietnamese flag T-shirts, red with gold stars. Much of the center of the city was blocked off from traffic as crowds swirled as close as they could get to the tightly guarded parade route beneath signs and banners with the numerals “50,” for 50 years since “Liberation” from the American-backed regime.

An American visitor encountered no smoldering anti-American feelings despite repeated allusions to American “imperialism” in speeches by the general secretary and others before the marching began.

“The U.S. imperialists quickly replaced the French colonialists, intervening in Việtnam, carrying out the plot to divide our country, and turning the South into a new type of colony and an outpost to prevent communism,” he fulminated, according to the official English translation. “The U.S. imperialists deployed a large number of troops with the most modern and sophisticated weapons, implemented many dangerous war strategies, carried out two brutal destructive wars against the North, causing a lot of pain and losses to the people in both regions of the country.”

As he droned on, one woman said she could not blame the American ambassador for staying away.  “They insult America but want America to help them against China,” she said through an interpreter. “I had to stop listening to what he was saying.”