I AM AFRAID that I shall never see Erika again. The authorities in Czechoslovakia have put an end to our friendship, the friendship of two women, one a Czech citizen living in Prague, the other an occasional visitor from the Unitd States.

They did this with chilling efficiency a few hours after my arrival, telling me that because I was "making contact with dissidents and Charter people" I must "leave the territory of the Socialist Republic of Czechoslovakia immediately" or I would have "great problems."

Erika (not her real name) was the only person I had seen so far in Prague and she is, admittedly, a "Charter person." Six years ago she signed a document known as Charter 77 which called on the Czechoslovak government to honor the human rights of its citizens.

More than 1,000 citizens in Czechoslovakia signed the Charter and each of them has paid a price for that act. Charter signatories have lost their jobs, their homes, their driver's licenses, their telephones. Their children are denied entrance to universities. They live under strict police surveillance -- of their movement, their conversations, their mail.

Some have been attacked and beaten up in the streets; others have been arrested and are serving long prison sentences. Many have been forced into exile.

As recently as Jan. 5, one of the current spokespersons for Charter 77, Ladislav Lis, was arrested and reportedly is being held on trumped-up charges. Yet the Charter signers do not consider themselves dissidents. As a Charter spokesman told me during a previous visit to Prague, "Dissidents are people who oppose the will of society, while we repesent the aspirations of 90 percent of the people. It is our leaders who are the dissidents."

On the first morning of what ws intended to be a four-day stay in Prague, I went to see Erika. She seemed much too thin, I thought, smaller than I had remembered her, but fragility also accentuated her wide green eyes and lovely face. Wearing jeans and a loose-fitting shirt, her long, gray-blonde hair pulled back into a bun, she was apparently cleaning her house when I appeared at the door. She immediately offered tea in her small kitchen, "real English tea, the best in Prague."

Erika is a recent widow, the mother of three young sons, none of them yet out of their teens. Her husband had been a writer whose controversial works could only be published abroad. Her husband from an earlier marriage, also a writer, now lives in exile in London, stripped of his Czech citizenship while abroad. Her parents, both officials in the Dubcek government during the brief, exciting "Prague Spring," were forced to leave Prague when Soviet troops invaded the city in 1968 and have living in Paris ever since. She had not seen them for 15 years.

The isolation in which Erika and othe Czech intellectuals live is almost tangible; it hanges heavy in their homes and in their words. Deprived of books and ideas from the outside world, ostracized by society, they cling to each other for moral support.

"I never greet anyone from my past when I see them in the street," Erika confided. "I wait to see whether they will say 'Hello' to me or pass by as if we had never met."

She spoke with sadness about the fact that her children will not be allowed to enter college: "They are paying this price for their parents' decisions. That is not fair." She spoke of increasing loneliness as more and more of her friends emigrate to the West. Yet she herself stands firm: "I would love to travel abroad," she said in cultivated, almost unaccented English, "and to live there for a year or two, maybe even 10. But only if I knew that I could come home when I wished. When Christmas comes, I will always want to be in Prague."

Several years ago, Erika and I had set out together to visit the historic Jewish museum and cemetry in the old city, only to realize that it was Saturday and the museum, of course, was closed. It was then that we each discovered that the other was Jewish -- and that we both should have known that the museum would be closed on the sabbath. Now, reliving our friendship, Erika reminded me with amusement of that abortive visit and suggested that we try again, this time on a weekday. We made a date to do so later that very day.

Several hours later, however, on my way to meet Erika, I was stopped by the police and ordered out of the country. "Go straight to the hotel, pay no visits, pack your things and be over the border by 10 this evening. You are under control."

"Under control" meant that I was then followed openly, no longer covertly, by three plainclothesmen in a yellow, unmarked car. The car stayed close behind me until I had driven at least 30 miles out of Prague. Then, apparently convinced that I was following directions, it turned suddenly and roared away.

The night was clear, the road deserted. A full moon lit the snow-covered fields and gave a slight patina to the thin, icy coating on the highway. I passed through a small village, unchanged by time, quiet as a ghost town, not a person in sight. It was just a few minutes before 10 when I reached the Austrian border.

The barrier that was eventually raised to let me pass into the customs area was made of iron so thick and heavy that no car or truck could crash through. The customs officers are on the lookout for ideas and people, the first to keep out, the second to keep in. When I had entered the day before they had studied my papers, notebooks and address book with great care. Now, as I left, they checked the car itself to see that no one was hidden in the trunk, under the seats or even hanging under the car in a desperate effort to escape.

Along the border, two parallel lines of barbed wire and electrified fencing stretch off to infinity, the lines coming closer but never touching, like an art book illustration of perspective. Between the fences there's a no- man's-land, heavily mined.

Twenty minutes later I would be in an Austrian village, the architectural sister of the Czech village I had just passed through, but full of light and life. There, in a cafe, I would drink beer and eat goulash, the same substantial fare that one finds in Prague. Sounds would be suddenly dear to me: the sounds of laughter and high-spirited conversation that are never heard in a public place in Prague.

At that moment, however, there at the border between East and West, I was looking backward, overwhelmed by sorrow for the friends in Prague that I had not been able to see, for Erika all dressed up for our afternoon's outing, waiting with happy anticipation for a friend who never showed up.

I said a quiet farewell to Prague, that heartbreakingly beautiful city whose hundreds of years of culture and civilization have culminated in a system so absurd and barbaric that I had been afraid, for Erika's sake, to call on the phone, just to say: "I'm sorry. I cannot keep our appointment today."