BUENOS AIRES, Argentina (AP) _ Marcelo Lucero was caught red-handed. A policeman watched as the young man, eyes on the loot inside, broke into a parked car just before dawn.

It was a routine case, but when it came to trial in October, the courtroom was packed.

Lucero, 22, was the first person tried under a revised criminal code that allows journalists and the public to attend trials, and gives defendants rights that are common elsewhere. They can be present in court, confront their accusers and speak directly to the judge, but there still is no trial by jury.

''I always thought that, if in 100 trials oral arguments convinced me one single time ... that 1 percent is enough'' to warrant changing the 103-year- old code, Justice Enrique Petracchi of the Supreme Court said in an interview.

Before the revision, prosecutors and defenders presented virtually all evidence in written briefs and there was no tradition of accused confronting accuser. Defendants sat in jail during their trials and rarely, if ever, saw the judge.

''The judge didn't know the person he was judging,'' said Luis Dobniewski, a lawyer. ''I would say more: He wasn't judging people, but files, which sometimes he didn't have time even to read properly. That task ... was delegated to clerks.''

Pressure to revise the code came from political parties, human rights groups and bar associations.

Leon Arslanian, a former justice minister, steered the legislation through Congress. He said the new code, which took effect Sept. 5, ''will allow for quicker trials ... a better defense ... and a notable improvement'' in the Argentine judicial system.

It applies to all crimes committed in Buenos Aires and to federal crimes committed anywhere in the country.

Justice Petracchi said the changes represent a step forward for Argentine democracy, which is still recovering from the military dictatorship of 1976-83, the sixth since 1930.

''People can come and see what judges do,'' he said. ''This produces a catharsis. When someone identifies with the criminal, the district attorney, the defense, the judge, this ... is a form of participation.''

Under the old, closed system, too many people saw court procedures as ''mystic rites'' rather than the rule of law, he said.

Lucero, during his routine but historic trial, listened stoically as the policeman who arrested him described the events, then had a chance to tell his version to Judge Miguel del Castillo.

Television cameras and newspaper photographers recorded the scene. Curious onlookers seated on wooden benches at the rear of the old, high-ceilinged courtroom leaned forward to hear testimony.

Del Castillo declared Lucero guilty on the spot, but sentenced the first offender, who wore jeans and a Notre Dame sweatshirt, to time served: the 46 days he had spent in jail awaiting trial.

''This would have taken six to eight months more'' under the old system of doing everything in writing, the judge said.

Other changes include a new court to handle appeals and make sure the code is implemented correctly. Some criminal cases will be heard by three-judge panels instead of single judges. Hundreds of new judges will be appointed.

Judges now have the discretion to base a decision on testimony from a single witness. Before, testimony not supported by a second witness could be used only in special cases.

Trial by jury, an issue that has long divided the Argentine legal profession, was debated but not adopted.

''No one should fear trial by jury,'' said Jorge Andres Garlan, a lawyer. ''It's in the constitution and there aren't good arguments against it.''

Ana Maria Ladeuix, a colleague, had reservations: ''I don't see in our people the culture or custom required to apply a trial by jury. It seems better to me that the determination of guilt or innocence be left to judges.''

The new criminal code also revises police procedures. Officers now must advise prisoners that they have the right to call a lawyer and to remain silent until the lawyer or a judge arrives.

''It's like in the (American) movies,'' an inspector grumbled. ''We have to say, 'You have the right not to talk, and if you do, we can use that against you.'''

A requirement that they notify a judge of an arrest within six hours appears most upsetting to police. The old code allowed 48 hours for checking criminal records.

''Police claim ... and maybe they're right ... that in other countries, that's easy'' because of computers, said Petracchi, the Supreme Court justices. In Argentina, most police data is handwritten on file cards.

Petracchi said the restrictions were not especially aimed at reining in the police, although they have been accused of coercing confessions, using undue force and abusing their authority.

The Permanent Assembly on Human Rights printed pamphlets setting forth the rights of citizens under the new code and distributed them free.

Graciela Fernandez Meijide, a spokeswoman, said she believed the new code would ''limit the possibility of arbitrary police actions.''