Soviet military reservists in the strategic Carpathian military district bordering Poland were activated in great haste in August and remain on duty, according to accounts reaching here from the sensitive, closed region.

These unofficial sources appear to confirm reports circulating in the West in September that reservists were activated in Soviet regions near the Polish border. But there has been no indication until now that reservists have been kept on active duty.

The sources assert that a large but unknown number of reservists under age 35 have been told they will stay on active duty at least through the end of 1980. The Carpathian area, with a major headquarters in Lvov, close to the Polish border, was an important staging area for the 1968 invasion of Czechoslovakia, which the region also borders.

A Soviet Foreign Ministry spokesman flatly denied the report, which could not be independently verified. "There has been no urgent mobilization of the reserves and no expectation in this respect," he said.

But informed foreign sources here say the reports, rich in detail, are consistent with recent irregularities believed seen in Soviet troop rotation patterns, and with other signs of increased readiness among units stationed in western Russia. The State Department said last week that Soviet troops near the Polish border had been put on a higher state of alert than usual, and that regular units had increased manpower to full strength. But the department spokesman, John Trattner, said there were no signs of plans to invade Poland.

The reports reaching here from the unofficial Soviet sources who cannot be identified, tend to support that view. They say an air of emergency that gripped the region in late August when the Polish strike to demand an independent trade union movement was at its height has subsided and a number of unneeded reserves have been released to civilian duties.

But, these sources insist, the original call-up 3 1/2 months ago occurred in a state of extreme urgency, with police stopping trucks and private cars and sending them to collection points where drivers and vehicles were given assignments by military specialists.

Reliable foreign sources here said similar abrupt tactics were used to commandeer trucks before the Soviet invasion of Afghanistan last December.

Reservists are said to have been activated in the class one category (up to age 35) with many given no opportunity to seek waivers.

Warsaw Pact states held publicly announced maneuvers in East Germany's Baltic Coast region, on the other side of Poland, in late August, but there was no mention then of maneuvers or call-ups in the Carpathian region.

Foreign sources emphasize that a reserve call-up does not mean an invasion will follow. It is viewed by these sources as nothing more than the minimum prudent precaution in case military force might eventually be applied in a situation that required massive troop movements.

There has been no mention of a mobilization or call-up in the Soviet press, unlike the days preceding the invasion of Czechoslovakia.

The unofficial sources say groups of older men, many clearly in lax physical condition and sometimes with incomplete uniforms pulled on over civilian clothes, can be seen in public transport depots in the area.

Soviet reservists normally receive one month of refresher training every five years, but they can be called to duty for periods of up to three months, a period that now apparently has been exceeded.

The Carpathian region is generally closed to foreigners, except for Lvov and several smaller cities as far south as Chop, the principal crossing point into Czechoslovakia and Hungary. Western military attaches have been able to arrange only a few trips into open Carpathian cities in the past five months.

The Soviet sources say some reservists have been instructed in truck convoy methods and that in late summer, lines of dozens of vehicles could be seen on the highways, although lately this is said to have all but ceased.

The Carpathian military district is one of 16 in the Soviet Union and is thought by some sources here to command about seven regular ground divisions plus rocket forces and air support.A Soviet motorized rifle or armored division numbers about 13,000 troops.

There have been no reports here of similar reserve call-ups anywhere else in the Soviet frontier regions. It is thought here that the most promising line of advance for Soviet forces into Poland would be across the flat Baltic plain west to Gdansk and south to Warsaw.

The Carpathian region, now part of the Soviet Ukrainian Republic, has changed hands several times in past centuries and was formally ceded by Czechoslovakia to the Soviet Union in 1945. Its population includes major groups of Hungarians, Czechs and other peoples with strong ethnic ties to the socialist countries to the west and south.