Life in Saigon, three years after its fall and rebaptism as Ho Chi Minh City, is marked by harsh misrule, political oppression, acute food shortages, economic mismanagement and rampant official corruption, according to four French journalists who recently spent 10 days there, gaining the most extensive access of any Western reporters since 1975.

Granted visas to report on "Cambodian atrocities," the French reporters - three of whom had written sympathetically about North Vietnam during the war found much more to report about the Hanoi regime's "disastrous" policies in what used to be South Vietnam.

In the view, Vietnam's leadership, molded by a lifetime at war, has been unable to adjust to peace.

Facing antagonism from China andhampered by crop-destroying floods, the "aging Communist leaders in Hanoi see only one solution: more enforced socialization, more confrontation; more repression," the correspondent for Le Nouvel Observateur wrote.

The main impact of Hanoi's tough policies is in the south, where North Vietnamese officials "behave like an occupation force . . . while Saigon's inhabitants are cowed by hunger," LeMain's correspondent wrote.

These views were corroborated by the reporters from Le Point and Le Monde. The report in Le Monde provoked an editorial about the "Vietnam gurag."

By eluding official supervision, the four journalist managed to make contact with a number of South Vietnamese including former acquaintances. In their articles and in subsequent interviews, they agreed that conditions there are grim.

In Ho Chi Minh City, all key positions are held by North Vietnamese officials, all Communist party members whose importance can be roughly ranked on sight by the number of ballpoint pens in their open-necked shirts - the only insignia they can afford," the Le Point correspond said, only half-jokingly.

The northern officials move about the city in curtained U.S. sedans and live inrequisitioned villas, whose garden walls have been topped with wire barriers. "There is more barbed wire in Saigon today than under Thieu,"said a reporter who had covered the last days of South Vietnam under premier Nguyen Van Thieu.

The former U.S. Embassy has been converted into the central security headquarters and prisons are undergoing expansion. The reporters were denied precise statistics, but they claim to have received information substantiating Vietnamese refugee claims that there are more political prisoners today than there were under the Thieu regime.

Thousands of South Vietnamese including many former Viet Cong fighters continue to disappear, the reporters said often without trace, into reeducation camps," usually agriculture work camps lacking adequate nutrition or medicines.

One million South Vietnamese have been 'rehabilitated and released," Hanoi spokesman contend, but the human rights issue has caused the European Economic Community to hold up $4 million worth of proposed aid for a Vietnam resettlement project.

South Vietnamese who had expected to be able to cooperate with the new regime in 1975 have found their hopes dashed. Buddhists and liberal journalists who opposed the Thieu regime are said to be in prison again, this time at Hanoi's order.

"The only difference is that the arrest orders come from the Communist Party, not Thieu's courts," a reporter said, "and neutral observers are no longer allowed to inspect prison over-crowding."

Hanoi has confirmed reports by Vietnamese refugees that the Buddhist leader Venerable Thich Thien Minh had died in reeducation camp 100 miles from Ho Chi Minh City. Thien had led opposition ti successive regimes in South Vietnam.

The reporters had a brief meeting with Huynh Tan Phat, head of South Vietnam's short-lived provisionalrevolutionary government. He was quoted as having said: "Officially we were separate [North Vietnams and the Viet Congs], but in fact we were the same thing all the time; there was was a single party all the time; there was a single party, a single government, a single capital, a single country."

This confirmation of Hanoi's historical control was all the more striking because Phat in the past had ardently maintained that North Vietnam would never "swallow" the south and would allow a three-year transition period - which, in fact, ended only a year after the capture of Saigon.

The meeting with Phat provided a rare glimpse of a former Viet Cong official. AS usual, the reporters were handed a paper containing stereo-typed replies to questions that they had posed in advance, and their North Vietnamese escort said afterwards their account should be restricted to the written interview, ignoring Phat's oral remarks.

In Ho Chi Minh City, "whose streets City down to size, literallY, by reducing the population from 3.5 million to perhaps 1 million.

In an effort to drive Saigon residents into the countryside, commerce has been nationalized and inefficient ration system imposed. South Vietnamese are denied jobs commensurate with their skills. The reporters found that Cholon, the once - bustling Chinese quarter, has emptied about half of its million ethnic Chinese by truckloads in nighttime raids organized by the authorities.

However, the Hanoi-dictated farm policy reported has backfired because of forced colleectivization, which provoked a silent peasant rebellion.

Irate peasants have started to sabotage their rice crops rather than raise it to sell it on the low-paying official market. Pig-rearing, another traditional Vietnamese activity, has been prohibited in some areas in an unavailing effort to force more rice onto the market after farmers started feeding it to their livestock.

The food crisis - triggered by the end of Chinese aid to North Vietnam and U.S. aid to South Vietnam - has worsened because of severe flooding in the Mekong Delta and epidemics of pests due to new fertilizers. Instead of a green revolution Vietnam last year had a rice shortage of 2 million tons. The figure has doubled this year. The lack of food has become the overriding national issue.

These hardship conditions are giving rise to a rapid revival of corruption, the reporters agreed. To leave by boat, an ethnic Chinese must pay about $3,000, of which less than half reaches the government coffer. The rest goes to semiofficial middlemen, mostly North Vietnamese party officials.