August 29, 1981

John W. Hinckley Jr. pleaded not guilty today to charges of attempting to kill President Reagan and assaulting three other men last March. Mr. Hinckley announced his plea in a firm, clear voice as he stood before Judge Barrington D. Parker in a heavily guarded courtroom for arraignment. With the concurrence of prosecutors and defense attorneys, Judge Parker signed an order declaring that the 26-year-old defendant was mentally competent to participate in the arraignment and related proceedings in Federal District Court here. Mr. Hinckley ''has a rational and factual understanding of the charges against him and is able to assist his attorney in the preparation of his defense,'' the order said. Deadline for Insanity Plea The judge gave the defense attorneys until Sept. 28 to file a formal notice if they intend to use an insanity defense or to introduce expert testimony to the effect that Mr. Hinckley had some mental disease or defect. No trial date was set.

Mexico and France recognized El Salvador's guerrilla-led opposition today as a ''representative political force.'' The joint declaration, handed this morning to the current president of the United Nations Security Council, Jorge Illueca of Panama, runs counter to the Reagan Administration's policy. Washington seeks the military defeat of the guerrillas and urges democratic opposition parties to take part in elections for a constitutional assembly scheduled for next March. The United States was not mentioned by name in the statement.

Secretary of State Alexander M. Haig Jr. accused the rebels in El Salvador today of engaging in ''straight terrorism'' and again accused Cuba and the Soviet Union of playing major roles in fomenting the country's civil war. ''We've witnessed the guerrilla movement resort to straight terrorism, the kind of activity which reflects their failure and frustration in major force operations,'' he said. ''They have now gone into an effort in which the main victims of their activity are innocent noncombatants, the people of El Salvador.'' He referred to a campaign in thecountry to destroy power lines and disrupt water supplies.

Lieut. Arthur Dallas's longest day in the Police Department began when he fired two shots from his service revolver at a man who was waving a pistol at motorists on a South Bronx Street in June. The man, James Robertson, was killed, and the pistol turned out to be a plastic toy. ''I felt lonesome,'' Officer Dallas said later. ''You recognize you are out there on your own and you made a decision in a life-and-death matter. For policemen, this is the difficulty of the job precisely.'' Though there have been a number of such controversial shootings by police officers this year, both officials in the department and its critics say the New York City police show more restraint than the police in other major cities.

The Justice Department reversed itself today and said that it was satisfied with this city's proposals to desegregate its schools. On July 21, the department assailed the school board proposals for desegregating the nation's second largest school system as ''incomplete.'' The Cabinet agency said the city's plan placed an unfair burden on black students in that it would leave many schools as much as 70 percent white when white students make up only 18 percent of total enrollment, and would leave about 250 schools all-black or all-Hispanic American. Misunderstandings Suggested But in a joint statement submitted to the United States District Court, which is overseeing desegregation efforts here, the Justice Department did an about-face. It suggested that its criticisms had been ill-advised and the result of misunderstandings between the Government and Chicago school officials.