W HEN a senior Cambodian government official last week demanded that all mobile telephones have their power shut off and removed from the table before relaxing over a dinner conversation, he wasn't necessarily being paranoid.

For the same reason, the US Embassy now has its security guards confiscate all mobile phones before you are allowed to enter the premises. Security experts say that the phones can be used as listening devices, passing on conversations to spies with recording devices.

And sophisticated computers meant to compile bills are busily recording and identifying the origin of all incoming and outgoing calls made from many phones now operating in the country. The phone records are being seized by government security services, executives of several private phone firms in Cambodia acknowledge.

It is all part of a concerted campaign of monitoring telephones and fax machines, stepped up since the failed July coup, as officials continue to probe suspected plotters still in the ranks.

But the government has now widened its new-found ability and is using it in an aggressive crackdown against government critics in general. Targets now include journalists, UN officials, human rights organizations, diplomats, businessmen, and even other members of the Royal Government, according to officials, foreign intelligence operatives and communication experts.

Many government officials, human rights investigators, diplomats and journalists now refuse to use their phones to gossip or hold vaguely sensitive conversations, with people exchanging prearranged code names used only to set up meetings to talk in person. The use of phone records by government security staff has sparked a fear of guilt by association as officials fear they will be linked to cooperating with journalists or human rights investigators as the government widens its monitoring of perceived opponents.

The practice seems to be in violation of the Constitution. Article 41 states: "The rights to privacy of residence, and the secrecy of correspondence by post, telegram, fax, telex, and telephone shall be guaranteed."

But when a foreign journalist inquired with the Minister of Interior last week over reports that the assassination of a local newspaper editor was thought to be politically motivated, Minister You Hokry erupted with anger. " Who has called you and said this?," the minister was quoted as demanding. When the journalist declined to divulge his source, the Interior Minister responded:" I will go to [Cambodia] Samart [Communication Co] tomorrow and find out who these people are who called you saying this."

The next day an Interior Minister three-star general appeared at the news office with a computerized list of numbers of people who had called the news office that day, inquiring for further details.

The Ministry of Interior has approached each of the private phone companies operating in Cambodia and demanded they turn over computerized records of mobile phones owned by people targeted by the government, according to government officials, diplomats and company executives.

While some of the companies initially balked at violating the understanding of confidentiality they have with private clients, they say there is little they can do. In one case, the Ministry of Interior threatened to send in armed troops and forcibly confiscate the records if their demands were not complied with voluntarily, according to one company official, who asked to remain anonymous.

The official was backed by Anil Chandra Adhikari, general manager of Cambodia Mobile Telephone Company who said: "Many times this has happened to us."

Adhikari added: "When they come with a letter from the Ministry of Interior and say they want the details of this number and who called in and out between this time and this time, there is no way out. I give it to them, I have no choice."

He added this was not his biggest problem. "Our main problem is bad debt - mostly from government VIPs," he said. Adhikari said that his company, which uses Motorola equipment and is the second largest mobile phone company in Cambodia, has "more than $1 million in bad debt, more than half of that owed to us by the government. The Ministries guarantee the local and international calls, but they never pay," he lamented.

Cambodia Samart Communication Co, the biggest phone company in Cambodia said that 55 percent of their bills have never been paid since they began operation in 1992. Most of the bad debtors are government and military officials.

"Many times, if we shut off a phone for non-payment, they just walk in the office with AK47s and say 'what do you think your doing. Turn my phone back on now' ", one senior Samart official said, "What are we supposed to do? We turn the phone back on."

Communication and security experts say that government eavesdropping takes several forms in Cambodia.

Voice monitoring of landline phones is easily conducted with "bugs' attached directly on the phone, neighborhood "cabine" or at the origin of the line at the government controlled Ministry of Post and Telecommunications. Fax machine monitoring is even more efficient, with the government able to attach a line that diverts an extra copy of the fax directly to a fax machine at a government location.

Voice monitoring of mobile phones is commonplace also. Scanning equipment, available for several hundred dollars at the local market, can cover the less than 200 channels used by the phone and, when a intelligence official finds a conversation interesting, he can activate a tape recorder.

One foreign resident of Phnom Penh recalls having a drink with a senior government official recently, when the foreigner's mobile phone rang. While conversing, the government official smiled, pulled out a small handheld radio unit, and proceeded to broadcast his companion's phone conversation. Such equipment is available in many local electronic equipment outlets.

More sophisticated computer equipment can automatically turn on a tape recorder whenever a targeted mobile phone is being used. Every mobile phone has a ID chip with a code number. If that number is known, the computer can be programmed to monitor and record all incoming and outgoing calls automatically, communication security officials say.

But the method most preferred by the government is the computerized sheets of all records of incoming and outgoing calls to a certain number. These records, while not provided to the customer, list the numbers called and the numbers that call into every one of the approxamately 6,000 phones in Cambodia and are kept conveniently in a set of 40-50 computer tapes. Government security officials acknowledge that they scrutinize these records of suspected critics.

Phnom Penh-based diplomats say that the Cambodian Ministry of Interior employs 800 people solely to keep track of the activities of foreigners in Cambodia.