MANILA -- Singapore's Prime Minister Lee Kuan Yew, who has presided over more than a quarter-century of dynamic economic growth in his prosperous island city-state, has come under fresh criticism for his often harsh treatment of political opponents and his intolerance of almost any form of dissent against his rule.

Foreign and Singaporean observers say the issue of human rights and political freedom threatens to cloud Lee's otherwise impressive record as Singapore braces for a potentially trying time of economic retrenchment and its first transition of power since independence.

Most of the recent criticism centers on the May 21 arrest of 16 Catholic social workers, political activists and a prominent leftist human rights lawyer under Singapore's tough Internal Security Act. Under the act, individuals who are deemed a threat to national security can be held indefinitely without trial.

The government accused the 16 of infiltrating church organizations as part of a conspiracy to set up a Marxist state. The government has said that some of those arrested -- including the alleged ringleader -- have signed confessions, and it released excerpts of letters from one of the detainees to bolster its accusations.

But the government has refused to press its charges in court, and Lee has indicated in published remarks that no such trial is in the offing.

"It is not a practice, nor will I allow subversives to get away by insisting that I have got to prove everything against them in a court," Lee was quoted saying.

The case has provoked angry comment from opposition politicians and international human rights groups. They say the arrests, and a recent press crackdown, are part of a concerted effort to stamp out all forms of opposition to the ruling People's Action Party (PAP), as Lee prepares for local elections and for the possible naming of his successor if he steps down next year as he said he might.

The arrests also have split Singapore's Catholic church, with 10 of the detainees accused of using church groups as part of their alleged conspiracy.

After first criticizing the arrests, Archbishop Gregory Yong met with Lee and reviewed the purported confession of Vincent Cheng, executive secretary of the church's Justice and Peace Commission, who the government has alleged was the leader of the plot.

Afterward, Yong announced that he would take the government's accusations "at their face value." His about-face led four priests to resign their positions from leading church organizations.

J.B. Jeyaretnam, leader of the small opposition Workers' Party, was quoted in press reports as saying, "The whole thing is laughable . . . . The PAP is saying that anyone who works against injustice deserves to be locked up."

Amnesty International, in a statement from London, said it believed the 16 detainees were "prisoners of conscience engaged in the nonviolent exercise of their beliefs." It asked the government either to release the suspects immediately or charge them in court.

An Amnesty spokesman, interviewed by telephone in London, said the group has received previous reports of Singaporean prisoners being mistreated. He said Amnesty believes that the reported confessions of some of the detainees in this case may have been coerced after "continuous interrogation" and intimidation.

"We are concerned about the continued use of the Internal Security Act in such cases," the spokesman said. "We regard the use of this law in Singapore as a violation of the basic right to freedom from arbitrary arrest, to the presumption of innocence, and to fair and prompt trial in accordance with international standards."

In Washington, Eric Schwartz, program monitor for the Asia Watch Committee, a human rights group, said by telephone, "This comes after a year in which the government has taken a series of actions to limit the range of political debate, including restrictions on the foreign press . . . . What this appears to be is an attempt to chill opposition activity."

Both rights groups pointed out that one dissident, Chia Thye Poh, has been held in jail for 20 years under the Internal Security Act and never has been charged, tried or convicted of any crime.

The recent tightening of control over dissent has taken place under a paternalistic and quixotic prime minister who has tried to shape the lives of the country's 2.6 million citizens with unusual crusades such as eliminating smoking and encouraging people to smile more often. Lee also has banned long hair and video games.

Jeyaretnam, the Workers' Party leader, lost his seat in Parliament last year after the government pressed a successful court case against him involving alleged false declarations on his party's accounts.

The government also moved ahead with new restrictions on the activities of the Law Society, Singapore's equivalent of a bar association. One of those arrested, lawyer Teo Soh Lung, is an elected member of the society's council and has pushed the group to oppose various government bills.

During televised hearings on the government's proposed revisions of the Law Society, Teo engaged in a heated exchange with Lee.

In the area of press freedom, the government effectively orchestrated the removal of Peter Lim as editor of the Straits Times Press group, publishers of Singapore's main English-language dailies. Under Lim, some papers in the group had drawn the government's wrath for criticizing Lee's decisions to restrict sales of Time magazine within Singapore and to allow a visit to Singapore by Israeli President Chaim Herzog.

The toughest part of the media crackdown over the last year was reserved for the foreign press. Besides restricting the sale of Time, Lee in February cut distribution of the Asian Wall Street Journal from about 5,000 to 400 copies a day. The ban on the Journal drew criticism from American businessmen and from the State Department.

In both cases, the publications had refused to run the full, unedited text of official government letters responding to stories Lee did not like.

In April, the government refused to renew the work permit of a correspondent for the Far Eastern Economic Review, a move that amounted to expelling him.