David R. Berkowitz pleaded guilty yesterday to all six of the "Son of Sam" murders that brought terror to parts of New York for a year and triggered the biggest manhunt in the city's history.

In a tightly guarded Brooklyn courtroom, the 24-year-old postal clerk went against the advice of his lawyers and pleaded guilty to killing six and wounding seven during the series of nighttime shootings with the same 44 cal. revolver. Most of his targets were young women.

The court hearing was startled by the disclosure by prosecutors that Berkowitz may also have set as many as 2,000 fires, most of them in the ar-son-ridden Bronx.

Records Berkowitz kept from 1974 through 1977 indicate that he set the fires and then reported them to fire officials using the name "Phantom of the Bronx."

His records include the location of the fire box from which the alarm was turned in, the address of the fire, the kind of fire department equipment that responded to the call, the time of day and the weather. Although most of the fires were in the Bronx , some were not far from the scenes of "Son of Sam" murders in other boroughs. The fires, which he would have had to have set at a rate of nearly 500 a year, included small blazes in trash cans and vacant losts as well as buildings.

Berkowitz responded with a string of "yes" answers as three judges questioned him separately about whether he was aware of the rights he was abandoning by changing his pleas to guilty. His lawyers, Leon Stern and Ira Jultak, had urged him to plead not guilty by reason of insanity.

Brooklyn Supreme Court Justice Joseph Corso led Berkowitz through his activities the night of July 31, 1977, when "Son of Sam" killed for the last time.

Corso asked why he left his Yonkers apartment and drove to Brooklyn in his yellow Ford Galaxie.

"To kill somebody," was the reply.

"Did you have any particular person in mind?"

"No"

The only hesitation in Berkowitz's soft spoken replies came when Corso asked whether he aimed first at Stacy Moskowitz, 20, or her date, Robert Violante, 20, as the two sat in a parked car.

"Stacy Moskowitz," he said after a pause.

Corso asked where Berkowitz aimed.

"At her haad," he replied.

Moskowitz's died from her wounds; Violante was blinded.

Moskowitz's family was in court-room yesterday, as were relatives of other "Son of Sam" victims.

Under a unique arrangement to save the state the costs of three separate court sessions, judges, district attorneys and court clerks from the Bronx and Queens traveled to the Brooklyn court to accept Berkowitz's guilty pleas.

Bronx District Attorney Mario Morola revealed Berkowitz's fire records.

"There's no question in my mind that he's a firebug," Merola said. Bronx officials are still comparing Berkowitz's records to theirs, but a spot check indicates that fires did occur on the dates and at the locations he listed. Merola said he does not yet know whether there were any casualties in the fires.

THe fires will not increase whatever penalty Berkowitz receives. The three judges set May 22 for sentencing.

Berkowitz was charged with second-degree murder in the six killings, which carries a 25-year to life sentence. By law, he cannot be given consecutive sentences for the six killings, so 25 years to life is the maximum he can get.

During the examination by Corso, Bronx Justice William kapelman and Queens Justice Nicholas Tsoucalas, Berkowitz stood, ringed by five court officers. Other security men were standing and sitting around the court-room lest, as one put it, "someone tries to be a Jack Ruby" - a reference to the man who killed President Kennedy's assassin.

Again and again, Berkowitz said he emptied his five-shot revolver at his targets.

"I shot them," he said unemotionally.

His purpose, he told each judge in turn, was "to kill them."

Berkowitz said he knew at the time that murder was wrong, but he was not asked to explain what motivated him. During the year he terrorized the city, young women who believed "Son of Sam" shot at long-haired women pinned up their hair or cut it, and many young couples abandoned traditional lovers' lanes.

Berkowitz made no mention of the "demons" he earlier said drove him to murder, nor did he repeat his claim that a dog belonging to one of his Yonkers neighbors, Sam Carr, gave him orders to kill people.

The killer gave himself the name "Son of Sam" in notes left at the scenes of some of the murders.

"What consequences did you expect when you pulled the trigger," Corso asked of the night Berkowitz fired at Moskowitz and Violante.

"To me or to them?" Berkoeitz asked.

"That I'd be arrested and put in jail," be then answered. "They would die. They would die."

Everyone entering the 260-seat courtroom had to pass through two-metal detectors, one of them so finely tuned that it picked up even the foil around a piece of chewing gum.

For each of his three appearances, Berkowitz was brought into the courtroom handcuffed, but his wrists were freed while he was there. He did not look at the crowd jamming the room as he entered and seemed determined to get through the proceedings as rapidly as possible.

"Yes." "Yes." "Yes." He kept replying until the questions were exhausted.

He was brought to the Brooklyn courthouse in a heavily armed motorcade, with a helicopter flying overhead cover, about 45 minutes before the first hearing began at 10 a.m.

Almost half the courtroom seats were occupied by reporters and artists sketching the scene for newspapers and television stations.

Among the reporters were Jimmy Breslin of the Daily News and Steve Dunleavy of the New York Post, who before Berkowitz was captured Aug. 10, both appealed to "Son of Sam" to surrender to them. Their articles were the most emotional in a torrent of press coverage of the killings.

Breslin is coauthor of a book, called a novel, on a 44-cal. killer in New York, which is to be published soon. Dunleavy got his most recent licks in on "Son of Sam" with an interview with pentecostal sister Ollie Smith in yesterday's post.

The Brooklyn evangelist has been visiting Berkowitz in confinement and told Dunleavy that he wants to become a pentecostal minister and "save other sinners."