John W. Hinckley Jr., a bullet-proof vest bulging under his suitcoat, yesterday told a federal judge in a firm, high-pitched voice that he is "not guilty" of the attempted assassination of President Reagan and the shooting of three others last March 30.

U.S. District Court Judge Barrington D. Parker, who will preside at Hinckley's trial, was told by prosecution and defense lawyers that Hinckley is presently mentally competent to understand the charges against him and to help prepare his defense. Hinckley has been undergoing psychiatric examinations since early April to determine his mental state now and at the time of the shootings and whether he was criminally responsible for his acts.

Defense lawyer Vincent J. Fuller told Parker that defense psychiatrists are still examining Hinckley and that no decision has been made whether to argue that he was insane at the time of the shootings and thus not guilty of the offenses.

Hinckley, 26, a drifter from a wealthy Denver family, appeared pale but relaxed during the 32-minute court proceeding that was conducted under extraordinarily tight security measures. Hinckley had been brought to the U.S. District Courthouse from the brig at the Quantico Marine Base in Virginia shortly after midnight yesterday and was held, alone, in the prisoners' cellblock there until the 10 a.m. hearing.

The elaborate security operation included a decoy caravan of police vehicles, with sirens blaring, that escorted a green van from the courthouse at 12:05 p.m., reportedly with Hinckley inside. Law enforcement officials later confirmed that Hinckley actually remained in the courthouse cellblock until 2:57 p.m., at which point he was taken to an undisclosed location in metropolitan Washington and flown in a U.S. Park Police helicopter to Fort Meade, Md.

Hinckley will be held there in an Army stockade, officials said.

Surrounded by deputy U.S. marshals, Hinckley, wearing a navy blue suit and new black shoes, walked into the courtroom carrying a brown legal folder. Two marshals stood close behind him as he listened to Deputy Court Clerk Elizabeth G. Flynn read the 13-count indictment against him.

"Sir, how do you today wish to plead to this indictment?" Flynn asked him.

"Not guilty," Hinckley promptly responded.

His father, John W. Hinckley Sr., a Colorado oil executive, and his mother, Jo Ann, were seated in the courtroom, which was packed with about 100 spectators. At one point, when his parents were pointed out to him, Hinckley turned from the defense table, quickly waved his right hand and smiled self-consciously.

Parker told Fuller to notify the court by Sept. 28 if he intends to raise the insanity defense. The prosecution will then have until Oct. 16 to respond to defense papers, Parker said. No trial date has been set.

Fuller also told Parker that "at Mr. Hinckley's request" the defense wanted to know what terms Parker would set to allow Hinckley to be released on bond pending his trial, a request that the government is expected to vigorously oppose. Parker scheduled a hearing on the bond question for Tuesday at 10 a.m.

Under the 1966 Federal Bail Reform Act, a judge must set a bond for a defendant unless there is a risk that the defendant will fail to appear at subsequent court proceedings.

The amount of a money bond is set by the judge, but can be challenged by the defendant as unreasonably high. The prosecution could argue that because of the severity of the charges in a case, no money bond or other conditions of release would be high enough to insure that a defendant would return to court.

A defendant's potential danger to the community, if released on bond, can only be considered in so-called capital cases, in which the crimes charged are punishable by the death penalty, which is not the case with Hinckley.

The indictment against Hinckley includes one count of attempting to assassinate the president and four counts of assault with intent to kill Reagan, White House press secretary James Brady, U.S. Secret Service agent Timothy J. McCarthy and D.C. police officer Thomas K. Delahanty. All five of those charges are punishable by up to life in prison.

Law enforcement officials have said that Hinckley's alleged attack on the president may have been motivated by Hinckley's desire to impress teen-age actress Jodie Foster. An unmailed letter found in Hinckley's hotel room the day Reagan was shot, addressed to Foster and signed with Hinckley's name, said "give me the chance with this historical deed to gain your respect and love."

Assistant U.S. Attorney Roger M. Adelman said during the court hearing that the government would present 20 witnesses, including five experts, to prove its main criminal case that Hinckley shot Reagan and the three other men on the sidewalk outside the Washington Hilton Hotel, where Reagan had just finished addressing a group of trade unionists.

That testimony would take about three to four trial days, Adelman said. If insanity is raised as a defense to the charges, Adelman estimated that the entire trial could take 10 to 15 days. In such cases, the trial usually comes down to a battle of expert psychiatric testimony about whether a defendant was sane and responsible for criminal acts.

Psychiatrists designated by the U.S. Bureau of Prisons have filed a report with the federal court containing their conclusions about Hinckley's mental competency. That report has been sealed by the court.

Parker said that on Tuesday he will also consider defense complaints that prison guards at the federal correctional institution at Butner, N.C., read Hinckley's personal papers in violation of his constitutional right to privacy.

In papers filed yesterday, the prosecution said that the staff at Butner "never" read any papers Hinckley kept in a folder titled "attorney-client material," although other writings he left elsewhere in his prison room there were examined.