A judicial commission investigating the shooting deaths by police of 19 blacks near this city last month has heard testimony revealing that, in the weeks prior to the shooting, police turned from normal riot control methods to lethal weapons and shoot-to-kill orders in their determination to put an end to black demonstrations in the troubled region.

The testimony, given by police officers involved in the shooting, came during the first week of the panel's hearings.

It has shown that six weeks before the March 21 fatalities, the police in eastern Cape Province, where there has been almost continuous racial unrest for three months, were given permission to switch from light birdshot to heavy buckshot in their antiriot shotguns. They also were authorized to open fire with 9-mm service revolvers on "selected and properly identified targets."

That authority came from Brig. Bert Wandrag, head of the police counterinsurgency forces at national police headquarters in Pretoria, on a temporary, 15-day basis. It was extended for another 14 days on March 6 at the request of the regional police chief.

One week before the shooting, the issue of tear gas, rubber bullets and birdshot, regarded as standard antiriot equipment, was stopped to antiriot patrols in the Uitenhage area.

After March 15, according to Lt. John W. Fouche, the officer who gave the order to fire on the crowd March 21, they were issued only 7.62-mm semiautomatic assault rifles and extra-heavy SSG shotgun cartridges, which, as the commission was shown in a demonstration, are capable of blasting half-inch holes in a heavy sheet of metal.

Under cross-examination, Fouche and other police witnesses said they did not know who had given the order for the change in equipment issued to them, but Fouche said he thought it had come from Pretoria.

Two days before the Uitenhage shooting, a general order was issued from police headquarters in Pretoria, with the knowledge and apparent approval of Law and Order Minister Louis le Grange, to "eliminate" anyone seen throwing an acid or gasoline bomb.

The telexed order, issued by Gen. Hendrik de Witt, second in command of the national police force, and addressed to all regional police chiefs, stated that "when acid and or petrol [gasoline] bombs are thrown at police vehicles, private vehicles and buildings, every effort must be made to eliminate the guilty parties."

The testimony has shocked even firm supporters of the government's get-tough approach to black unrest, and it is being widely predicted that it will lead to the dismissal from the Cabinet of the hard-line le Grange.

The tall, graying le Grange has been backed by President Pieter W. Botha until now, with the president taking the view that the government must act firmly against what he believes are black radicals trying to sabotage his policy of cautious reforms, but the evidence that le Grange apparently sanctioned deliberate retaliatory killing could change that.

Some senior figures in the administration already seem anxious to dissociate themselves from some of the damaging disclosures emerging from the inquiry.

Col. Adolf van Rooyen, the chief of police riot control, had little hesitation when he appeared before the commission in dismissing claims by the local police commander that tear gas and rubber bullets were ineffective, and saying that he found the decision to stop issuing them to antiriot patrols in the Uitenhage area "amazing."

The inquiry, which is likely to last another three weeks, has become a focal point in this dowdy industrial city about 25 miles north of Port Elizabeth.

Few of the local whites are following the proceedings, but several hundred blacks gather each day on the pavement outside the labyrinthine magistrates' court building where it is being held.

As a result, presumably, of a decision to prevent large crowds of demonstrators from attending, the inquiry is being held in a cramped, stuffy courtroom with only 50 seats in the public gallery. Every day, 50 blacks chosen by the township residents line up to take these seats. Afterward they report back on the day's proceedings.

Donald D. Kannemeyer, a provincial Supreme Court judge who was appointed by President Botha to constitute a one-man commission of inquiry, is a soft-spoken man with graying hair. He sits behind a high wood-paneled bench, decorated with a small effigy of the scales of justice that has one of its scale-pans missing.

There is no suggestion of one-sided justice, however.

Although there have been some questionable commissions of inquiry in South Africa in the past, lawyers appearing for the families of those killed say they are satisfied with the way Kannemeyer is handling this one.

There are conflicting accounts of what happened on the day of the shooting. Blacks who were in the crowd say they were walking peacefully and unarmed to a funeral in another black township on the far side of Uitenhage when the police in two armored personnel carriers called Casspirs opened fire on them without warning or provocation.

The police say the crowd was heavily armed and unruly, and was marching on Uitenhage singing a song in the Xhosa language that said that they were going to kill the whites living there.

According to a version given to Parliament by le Grange on the day of the shooting, the police opened fire when they were surrounded and attacked by the mob with sticks, stones and gasoline bombs.

There are serious discrepancies in the police testimony. Faced with evidence that there were no bloodstains on the road beyond the Casspirs, nor any signs of incendiary bombs having been used, Fouche said le Grange's account to Parliament was inaccurate: no gasoline bombs were thrown, and the police never were surrounded.

Fouche said that he gave the order to fire after only one stone was thrown and that many more stones were thrown after the shooting began.

Other police witnesses said Fouche gave the order only after many stones were hurled at them.

All have had difficulty explaining why a police photograph taken immediately after the shooting shows no stones or other missiles near the Casspirs.

The stones were mostly round and must have bounced away, Fouche suggested. This prompted a sarcastic retort from the judge: "So these suicidal people were being hit by your bullets and by their own stones, which were bouncing back on themselves."

Another policeman, Sgt. Gerhard Stumke, was asked why no policemen were hit in the hail of stones. "The Lord was watching over us and protecting us," he explained.

All the police witnesses have said that, while no gasoline bombs were thrown, they saw the leader of the crowd, a man dressed in a long black smock who was the first to be shot, holding one. They have been unable to explain why neither the bottle nor fragments of it were found at the scene afterward.

According to those en route to the funeral, they were not being led by a man in black, but a boy on a bicycle was riding out ahead of them and was the first to be shot.

The police witnesses all have said there was no boy on a bicycle, but they have been unable to explain why a blue Raleigh bike is included in the inventory of items found at the scene after the shooting.