Allied officials at a new round of East-West troop reduction talks set to open here today privately claim that the Soviet Bloc "is either fiddling, cheating or lying" about the number of troops it has along the Central European front facing the West.

The biggest discrepancies, Allied officials say, are in the number of Soviet and Polish troops, as opposed to East German and Czechoslovak soldiers, that the Warsaw pact claims are in the region covered by the negotiations.

"How can they talk that way," counters a senior Communist diplomat here, "they have no roster" of our military units or personnel.

These opposing views concern what negotiators on both sides of the NATO-Warsaw Pact "mutual and balanced force reduction" (MBFR) talks call "the data issue."

In brief, it refers to a Western contention that the Soviet-led Pact has about 155,000 more soldiers in Central Europe than the Pact will admit to officially.

In a deeper sense, it raises the puzzling question for Western officials of why the Soviets are "fiddling, cheating or lying" overtly to the West in a field where Western intelligence is highly confident of its estimates.

The issue represents an enormous and possibly insurmountable obstacle that must be overcome if these long-stalled negotiations that have been going on for five years here are ever to yield an agreement that will reduce military forces, and hopefully tensions, along the most critical East-West border.

What makes this new round - the 16th since the talks began in October 1973 - so potentially crucil as a test of good faith is that it is the first since the Soviet Bloc submitted new proposals in June.

In those proposals, the Soviet Bloc appeared to agree, at least conceptually, to the long-standing Western position that there should be approximate parity and a common troop ceiling for both forces in the region. The Soviet Bloc agreed to a common ceiling of about 900,000 army and air force personnel on each side, with the key figure being a limitation on 700,000 ground troops.

These proposals were hailed, albeit cautiously, by President Carter last June as "a step in the right direction." The Soviets, he said, had now replied to earlier Western proposals "in a very affirmative way."

In the following months, however, it became apparent that the data supplied by the Warsaw Pact and the line being taken here by Communist officials indicate no change in the Eastern arithmetic.

The allies insist that the Warsaw Pact has 962,000 grounds troops and needs to withdraw 262,000 to reach the ceiling. The Soviet bloc contends it only has 805,000 ground troops and thus only has to withdraw 105,000 to comply.

The Soviets have not disputed NATO's estimate of 791,000 Western grounds troops, which means a Western reduction of 91,000.

In effect, the Soviets argue that there is already rough parity in the region. But Western officials say that the Soviets appear not to have budged at all from their long-held position that they will not give up the relative numerical superiority they have always held on the central front.

Some experienced Western negotiators believe that the real issue is not the data problem, but whether the Soviets will decide they want a troop reduction agreement. If they do, these officials aruge, than there are enough loopholes in the data question for Moscow to proceed with an agreement.

They insist that the West will not try to "finesse" an agreement by watering down its own figures in order to reach a compromise.

The 805,000 figure was first broached, with no corresponding detail, by the Warsaw Pact in June 1976. Since then Soviet President Leonid Brezhnev has stated publicly that the Warsaw Pact has not added any more soldiers in the field. Thus, the Soviets must stick to the same figure or suggest Brezhnev's statements were false.

Actually, Western delegates here say they could not really prove Brezhnev wrong. The Soviet build-up in recent years has been in equipment rather than men, they say, and the Pact forces were much larger to begin with before June 1976.

Officials in several Western countries insist this is an area where Allied intelligence is extremely good. "The whole of Western intelligence simply could not have been that wrong for the past 15 years," one official said.

Intelligence of East Germany is viewed as especially good because of closeness to the Western borders, heavy travel by West Germans in the East and the handful of Allied military liaison posts around Berlin.

The massive movement of Soviet forces into Czechoslovakia in 1968 and a steady flow of Czech defectors also helps also helps pinpoint data on that country. U.S. intelligence is focused heavily on Soviet forces.

Defectors, diplomats, spy satellites and electronic eavesdropping all contribute to intelligence that, along with equipment information, leads to final estimates of manpower.

As to why the Soviets would seek to confront the West with figures the West knew to be wrong, one experienced official reasons that it perhaps does not signal a harder Soviet line but is rather "a necessary stage that they must go through to convince themselves that we won't accept it."

Others believe the June proposals were born out of Soviet Bloc concerns that a new NATO long-term defense program, which could offset some Warsaw Pact strength, was moving ahead and that the new proposal might help stall that initiative.

Most here view the June proposals as a clever bit of progress, "superficially seductive," as one official put it.

The move recaptured the iniative from the West so that it is now a Soviet proposal that is the most recent one on the table. Western officials here say they presently have no new proposal to present at this new round but rather will press the Warsaw Pact on providing more details in an attempt to focus on the source of the data dispute.

A Communist official says that "there are possibilties to show good will and the data discussion is not completely sterile." He suggests that the West might be counting nonmilitary groups, such as railroad repair crews, or that it is making a mistake in assigning the same military manning labels to troops in the rear as those close to the front.

Westerners claim there are no mistakes and say only active-duty, nonreserve, troops are included in Western estimates. Still, the search for face-saving loopholes will go on.