Until last year, most Cambodians had lived under only one leader. Hun Sen ruled as prime minister for nearly four decades, tightening his iron grip over the country and systematically silencing the opposition, activists and independent media.

When Mr. Hun Sen appointed his oldest son, Hun Manet, as his successor, there was a sliver of optimism that civil liberties would improve. The new leader had attended universities in the United States and Britain, where he was exposed to a more liberal approach to elections and human rights.

But since he took power in August 2023, those hopes, however meager, have been dashed.

In recent months, the Cambodian authorities have locked up environmental activists on what critics say are trumped-up charges. They detained Mech Dara, a respected journalist, for nearly a month, in what was seen as an attack on press freedom. And they have cracked down on dissent from Cambodians overseas, securing deportations from Thailand and Malaysia.

“The space for expression has been at a low base, probably since 2014, and is shrinking further,” said Marc Thayre, the British deputy ambassador to Cambodia.

That sentiment was echoed by a United Nations official.

“We were hoping this next generation of government would be more liberal,” said Vitit Muntarbhorn, the U.N. special rapporteur on human rights in Cambodia. “Now it looks as though there is no sign of liberalization at all.”