The nation's major press organizations joined yesterday in denoun-cing what they characterized as the Reagan administration's efforts to curtail the Freedom of Information Act in the name of reforming it.

The opposition was led by the Society of Professional Journalists, which is holding its national convention here. In a welcoming address yesterday morning, Washington Post Co. board chairman Katharine Graham said the administration's proposals to overhaul the law, made public last month, are unjustified, unnecessary and undesirable.

Graham, who is also chairman and president of the American Newspaper Publishers Association (ANPA), said she is not suggesting that the 15-year-old law could not be improved, "if what we are really talking about is improving it and not weakening it."

On balance, she said, "no single statute has ever given the citizens of democracy a better window on their government. The Freedom of Information Act says to Americans -- and to the world -- that the business of government in a democracy is the people's business."

The debate continued yesterday afternoon at a Senate Judiciary subcommittee hearing where representatives of the Society of Professional Journalists, the ANPA and the American Society of Newspaper Editors were followed at the witness table by FBI Director William H. Webster.

He argued that changes in FOIA were essential and emphasized that top Justice Department officials in the Carter administration had agreed on that point. The FBI director said he feels special protections for documents pertaining to foreign counterintelligence, organized crime and terrorism are particularly important because "they are more vulnerable to analysis by those who have something to gain from trying to identify sources and to ascertain the scope and limitation of our efforts. Groups of individuals are free to pool our releases and subject them to detailed analysis."

The administration bill would permit the attorney general to decree what constitutes "foreign counterintelligence, organized crime, and terrorism" files, but Webster said he would be willing to work with the committee in defining those terms.

Webster said several FBI informants have reported confrontations with criminal figures claiming to have identified them by means of documents released under FOIA. He also said the fugitive Black Panther leader Joanne Chesimard left behind some 1,700 pages of documents provided her under FOIA when she escaped from a New Jersey prison in 1979. The FBI director said New Jersey authorities maintain she has been able to remain at large because of the knowledge she gained from those documents.

However, Jean Otto, past president of the Society of Professional Journalists, told the subcommittee that past FBI claims of a damaging impact on law enforcement have collapsed on close examination.

The administration proposal would accord far more secrecy to law enforcement records, business data and all types of government files "concerning individuals." Other witnesses who testified against it included Fredericksburg Free Lance-Star editor Charles S. Rowe, representing ANPA and the National Newspaper Association, and Dow Jones vice-president for news, Edward Cony, on behalf of the American Society of Newspaper Editors.