Despite Soviet warning - a quiet dissident works on

The gentlest Soviet dissident sits in the tiny study he shares with his books and house plants, and says: ''Let the police arrest me. I would not be against going to jail. . . .''

Historian Roy Medvedev, fresh from a high-level warning to cease ''anti-Soviet'' activity, speaks without bitterness or bravado. About himself, as about Stalinism (the major subject of his writings), he is always detached, quiet, academic, almost shy. He peers through thick glasses. His face is open and childlike, his hair gray and thinning.

He is 57 years old, 44 years since the Stalinist police took away his father forever, some 12 years since his twin brother was briefly confined in a mental ward, nine years since the brother was in effect exiled to the West.

Alone among major dissident figures, he has argued that Soviet reform can come quietly, from within, that the system will democratize itself, that pressure from the West is meanwhile counterproductive. He was once a Communist Party member and is still a Marxist. He has never been arrested or exiled.

He is the lonely, loyal opposition, seemingly protected in part by officials who think lonely, loyal opposition is needed. ''I have many friends,'' he says, ''in all areas of Soviet society.

''And I hope I shall be safe.''

Meanwhile, he suggests, he will keep writing for publication overseas books and articles, history and political commentary, that the authorities will not publish here. ''This is my life,'' he says. And at least so far, he continues to receive foreign journalists who stop - 15 minutes out Leningradsky Boulevard on the way to Moscow airport - at his small, nondescript apartment.

''Yes, I still believe reform of our system is possible,'' he says.

''People like to say our system is rigid. But in a way it is flexible. It was different under Stalin, under Khrushchev, under Brezhnev. It may be different under Andropov.

''But this reform comes only under pressure. There are two kinds of pressure: whether under rough economic conditions, such as in Poland or, presently, in China; or under the pressure of the people.

''Maybe none of this change will come quickly, maybe not until the 21st century. But reform will come, of this I have no doubt. . . .

''Maybe if the Soviet Union were the only country in the world, the system could stay perfectly unchanged for the next 20 years. But there are other countries, including America.

''There is, inevitably, competition, a process of keeping pace with economic change, a process that forces the people at the top to move ahead.

''And this requires political change, sooner or later.''

Mr. Medvedev believes he can play his own, small part in the process: not, he suggests, by forming or joining dissident groups systematically undone by the authorities, not by being ''anti-Soviet,'' simply by detached and untiring comment on ways in which the men who rule his nation fall short of the ideals they publicly espouse, by recording for the future his nation's past and present.

''I work alone. In our system, it is better to work alone.''

The warning from the Moscow prosecutor's office to opt for more ''socially useful'' activity has provided a reason for stock-taking.