The simplest way of telling when Poland's martial law authorities expect trouble is to visit half a dozen selected hotels in Warsaw. If they are occupied only by tourists, then it is safe to say that tensions are subsiding.

But if, as right now, the hotel guests consist of young men with greyish-blue combat fatigues, black boots and long, white nightsticks, it is a sure sign that the government is bracing for large-scale unrest.

To many Poles they seem like visitors from another planet, an impression that is reinforced by their strange-sounding name: ZOMO. Literally it is an acronym for zmotoryzowane oddzialy milicji obywatelskiej, or motorized units of people's militia. In other words, riot police.

It is on the ZOMO that the main burden of enforcing martial law regulations has fallen. They have stormed factories occupied by striking workers and broken up demonstrations in support of the independent Solidarity trade union. They certainly will be in action again Wednesday -- dubbed a national day of protest by the underground to mark the second anniversary of Solidarity's legal registration.

Of all the forces at the disposal of the military regime, the ZOMO are the most feared and certainly the most hated. Stories about them are legion. So are popular jokes about their alleged cruelty or lack of intelligence.

Question: "What do you get if you cross a ZOMO with a wolf?" Answer: "A very stupid wolf."

Or, "How do you help a seriously wounded ZOMO?" "I don't know." "Good."

Or, "Why do ZOMOs' faces always get scratched on Fridays?" "Because that's the day they are taught how to eat with a knife and fork."

More to the point, Poland's military ruler, Gen. Wojciech Jaruzelski, has been dubbed "General ZOMO-sa" by Solidarity supporters who regard the ZOMO as the single most important pillar of his political power.

In fact, the force that has inspired this awesome reputation is relatively small. According to Western and unofficial Polish estimates, the ZOMO number between 25,000 and 30,000 men. But their key characteristic is their high degree of maneuverability: They frequently are flown from one trouble spot to another to quell disturbances.

When major street clashes erupted in the southern steelmaking town of Nowa Huta last month, residents reported that planes carrying fresh ZOMO units were landing at a local airstrip every few minutes. They were brought to Nowa Huta from all over southern Poland, and their total strength was eventually believed to have reached around 10,000 men.

It was this mobility that led Solidarity's underground leadership to devise the tactic of coordinating protest actions nationwide. The theory is that if enough workers stage demonstrations in enough cities, the ZOMO will be spread so thin that they will be unable to cope.

The strategy has been only partially successful. There have been occasions over the last few months when the protestors have so overwhelmed the security forces that they have been able to hold their demonstrations unimpeded. But some incidents have ended in bloodshed with outnumbered police firing blindly into the crowds.

In order to create an impression of invincible strength, the authorities have supplemented the ZOMO with reservists. Known as ROMO, these frequently are ordinary farmers or workers drafted into the police on very short notice. They are dressed in exactly the same way as ZOMO but are used for guard duties rather than potential riot situations.

Because their lack of professionalism makes them easier targets, ROMO units are believed to have sustained more injuries than the ZOMO. In the Baltic city of Gdansk last month, several dozen ROMO who had been assigned to guard the Soviet Consulate had to be rescued by a mobilized force of ZOMO with armored personnel carriers and tear gas launchers after they were surrounded by stone-throwing crowds.

Describing his experiences as a ROMO draftee, a worker in Gdansk told a reporter that his unit was ordered to act as a backup for the ZOMO in manning a security cordon. "If the crowd breaks through, then run for your life as you won't have time for explanations," he quoted his commander as saying.

The fate of any militiaman caught by demonstrators is unenviable. According to a Polish newspaper account of last month's riots in Nowa Huta, a crowd of demonstrators tried to use the bodies of unconscious ZOMO as battering rams to break down the door of the local Communist Party headquarters. The report, in the respected weekly Polityka, did not make clear what happened to the ZOMO.

With the escalation of protests against martial law, reports of indiscriminate beatings of demonstrators by ZOMO have become more frequent. On occasions, as in Nowa Huta last month, groups of ZOMO have hauled workers out of buses or taxis well away from the scenes of demonstrations and beaten them up on the spot.

During a televised debate with Deputy Prime Minister Mieczyslaw Rakowski last weekend, one middle-aged woman accused the government of tolerating elements of "anarchy" within its own ranks. She said ZOMO had fired petards into private apartments, presumably in order to create an atmosphere of general terror.

The interior minister, Gen. Czeslaw Kiszczak, has acknowledged publicly that what he described as "incidents of abuse of physical power by militia officers" have taken place. But, in a recent newspaper interview, he argued that they were isolated and the men involved were subject to severe disciplinary action.

When they appear at the scenes of demonstrations, ZOMO units are routinely taunted by the crowds with shouts of "Gestapo," which, in view of Poland's experiences under Nazi occupation in World War II, probably is the worst insult here. According to underground Solidarity bulletins, the standard ZOMO training course now includes making the recruits attack human-sized rubber dummies while listening to chants of "Gestapo" over loudspeakers.

There have been several attempts to improve the ZOMO's public image. Meek-looking, bespectacled ZOMO officers have been interviewed on television and denied all intention of ever beating up anyone. Statistics have been published showing that nearly half the ZOMO troopers have secondary education and that 80 percent are voluntary blood donors.

Last week, the official mass media gave extensive coverage to the story of how ZOMO units in Nowa Huta were joining in the hunt for a child who had run away from home.

The hatred directed against the ZOMO has, however, had one welcome side effect for the authorities. It has deflected attention from the role of the 330,000-strong armed forces in implementing martial law. Compared with the ZOMO, the Polish Army is still regarded by most Poles as a patriotic institution, and the loyalty of its conscripts has not yet been put to the test.