PHNOM PENH, Cambodia— LATE last month, just before the international conference on Cambodia opened in Paris, Prince Norodom Sihanouk, Cambodia's former king and leader of the opposition coalition dominated by the Khmer Rouge, accused Prime Minister Hun Sen of being untrustworthy. ''You're a former Khmer Rouge,'' the Prince said.

''But you,'' Mr. Hun Sen responded, ''you're still the chief of the Khmer Rouge.''

Mr. Hun Sen's riposte was somewhat unjust, given Prince Sihanouk's distaste for the Communist guerrillas who brutally transformed Cambodia from 1975 to 1979 and killed hundreds of thousands of people, including many members of the Prince's own family, while keeping him under house arrest. Still, while Mr. Hun Sen tries to woo the Prince into abandoning the guerrillas and returning home to Cambodia as chief of state, for now, even as Vietnamese troops prepare to leave Cambodia by the end of September, Prince Sihanouk has chosen to remain in the chill embrace of the Khmer Rouge.

Yet in the quickness and self-confidence of his response, one can measure the growth of Mr. Hun Sen from a tongue-tied 27-year-old, installed by the Vietnamese in 1979 as the world's youngest and most inexperienced foreign minister, to an articulate and unintimidated prime minister of 38, whose deference to the 67-year-old Prince has its limits.

As pressure for a Cambodian settlement increases, the world has had to pay more attention to Mr. Hun Sen, the most visible spokesman for the Phnom Penh regime. As good a tactician as he seems to be, much about him is mysterious, including his basic beliefs. A military commander in the Khmer Rouge until 1977, the Hun Sen who now argues for private enterprise is at best a flexible pragmatist; at worst, a cynical charlatan.

Mr. Hun Sen is No. 3 in the hierarchy of the ruling People's Revolutionary Party, after Heng Samrin, the General Secretary, whose grim portrait in an ill-fitting suit adorns all state offices, and Chea Sim, the leader of the National Assembly. Neither sees Western journalists. But Mr. Hun Sen also retains the post of Foreign Minister, and it is an example of his increasing grip on the mechanisms of power that from the first of this year, all allied communication to any Cambodian official, even Mr. Heng Samrin, must first pass through the Foreign Ministry.

By requiring even the Vietnamese to go through proper channels, Mr. Hun Sen has been trying to show that he is Vietnam's ally, but not its puppet. Vietnamese officials expect the Cambodians to begin to insist on regular border checks, passports and visas for Vietnamese citizens, and do not complain.

''The Vietnamese have learned from the Chinese in this, too,'' a diplomat said. ''Influence works best if it is less obvious.'' 

Television Appearances

With an eye toward the possibility of internationally supervised elections, Mr. Hun Sen has also been pressing for a sharper distinction in responsibilities between the Government he heads and the People's Revolutionary Party, which retains a collective leadership within the Politburo. And in his constant appearances on television and visits to the provinces, he is acting more like a politician. In any future election, Mr. Hun Sen understands that his main competition would be Prince Sihanouk, the bulk of whose rule, before he was overthrown in a 1970 coup, is regarded by many older Cambodians as a time of relative prosperity and peace. Two years ago, when Mr. Hun Sen first met with the Prince, peasants in the countryside threw themselves on their knees before the televised image of their former god-king. Mr. Hun Sen basked in the reflected glory.

Since then, in his effort to bring the Prince home and prove himself a nationalist, Mr. Hun Sen has gone some way to liberalize what was a fairly orthodox, Vietnamese-style regime. In a series of constitutional changes, private property was protected, land rights provided to farmers and Buddhism restored as the state religion. But these changes were also made to insure that should the Prince return, he could not take credit for them.

To young people in Cambodia, who have no memory of Prince Sihanouk, Mr. Hun Sen represents modernity. Returning from Paris this month, he stepped off his airplane in a French double-breasted suit. He favors imported cigarettes and wears metal-frame glasses that help mask the scar from the shrapnel that took his left eye in 1975. Mr. Hun Sen takes advantage of his differences with the Prince while publicly urging him to come home. Meanwhile, he cleverly explains the Prince's failure to do so by saying that the Prince remains allied to the Khmer Rouge, a relationship most Cambodians find disturbing. About his own experience with the Khmer Rouge, Mr. Hun Sen says little, and his official biography is mysterious. He says he joined the Khmer Rouge in 1970, after the coup that overthrew Prince Sihanouk, because the Prince himself had sided with them and had called for resistance to the American-backed Government of Lon Nol. But the Australian scholar Ben Kiernan, in his 1985 book, ''How Pol Pot Came to Power,'' argues that Mr. Hun Sen was in contact with the Communists as early as 1967 when he fled the capital at 16 and worked as a courier for local Communist leaders before receiving military training. 

Split With Pol Pot

According to the Prime Minister's official biography, he rose to command a Khmer Rouge division and was wounded five times, losing his eye to shrapnel on April 16, 1975, the day before the Khmer Rouge entered Phnom Penh and began their radical transformation of Cambodian life. How far Mr. Hun Sen participated in these brutalities is not known, but it is hard to imagine that he stood aside. He became head of a military region in the eastern part of the country, where Mr. Heng Samrin and Mr. Chea Sim also served.

In 1977, in a growing rift with Pol Pot, who demanded increasingly self-destructive attacks on Vietnamese border villages, Mr. Hun Sen fled with other eastern zone leaders into Vietnam, escaping fierce internal purges. After Hanoi's invasion of Cambodia in December 1978, they were installed in power in Phnom Penh.

Both the Vietnamese and the Soviets say, hardly surprisingly, that Mr. Hun Sen was the brightest of that early core, a quick study with a forceful and analytical mind. How much of his early ideology he retains is unclear. When asked that question in an interview in May, Mr. Hun Sen said simply: ''Ideology doesn't matter right now. What the people want is peace and development.''

But will it matter later? Mr. Hun Sen isn't saying.