NEW YORK -- According to the columnist, Mayor Edward I. Koch is "behaving like a silly child throwing a tantrum."

According to the mayor, Sydney Schanberg is "immunized by pomposity," "tears people apart unmercifully" and "is one of the most unfair columnists I know."

The gloves are off. An acrimonious, long-running feud exploded into public view recently when Koch published an unusually personal attack on Schanberg in New York Newsday, where Schanberg has been a columnist since last year -- frequently attacking Koch on such issues as the city's homeless population, tax breaks for wealthy developers and the corruption scandals that have implicated some former Koch aides.

For his part, Koch, among other things, has questioned Schanberg's Pulitzer Prize-winning exploits in Cambodia, his subsequent resignation as metropolitan editor of The New York Times and the sudden termination of his Times op-ed column in 1985.

The mayor also accused Newsday of "censorship" for refusing to print one paragraph of an earlier missive against Schanberg -- a letter Koch published elsewhere in one of his six regular newspaper columns.

In the column that triggered the Koch eruption, Schanberg had chided the mayor for being "constitutionally unable to deal with criticism, other than by calling the critic a loony or a wacko or a pinko."

In an interview in his Third Avenue office, Schanberg, 53, dressed in a work shirt and blue jeans, said he was saddened by the Koch offensive but regards it as a price he pays for being a professional troublemaker.

"He does not see this, but he assaults his own dignity all the time," Schanberg says. "My reaction to that letter is that it speaks of him, not of me."

Schanberg says the letter dramatized his point that Koch has "poisoned the atmosphere" of New York politics by calling "everyone who disagrees with him some kind of bad name."

Across town in his City Hall office, Koch, 63, says he ignored his aides' advice that a politician should "never take on a reporter because they always have the last word."

"Week after week, he attacks me viciously," Koch says. "These guys get away with it because nobody ever takes them on. It's rare that the victim has an opportunity to respond. I'm different because I have six columns ...

"Do I have to be a bully to pick on Sydney Schanberg? ... This guy is a Pulitzer Prize-winning columnist, he's not some schmuck that I would just ignore. He walks in polite drawing rooms."

Sources say that David Garth, Koch's longtime friend and media adviser, called a senior Newsday editor and suggested that the paper spare the mayor embarrassment by rejecting the piece. But Editorial Page Editor Jim Klurfeld said that "we decided to let people see what the mayor is saying," even though he viewed the letter as unfair. Schanberg is fair game, said Newsday Editor Anthony Marro, because he is "a very strong voice" and "a personality in his own right."

The two combatants were not always slugging it out. When Schanberg ran The Times' local staff, he would ride in Koch's car during campaigns and lunch with the mayor and A.M. Rosenthal, then the paper's executive editor. "It was all very cordial," Schanberg says.

But when Schanberg began jabbing away as a pundit and Koch started counterpunching through the media, the sound of clashing egos soon drowned out all else.

Koch now even regrets throwing a party in 1980 to honor Schanberg and Dith Pran, his Cambodian assistant who, four years after Schanberg became the last American reporter to depart, had escaped the brutal Pol Pot regime. Koch now blames Schanberg for leaving Pran behind. Schanberg finds this "baffling," since he is the one who told the world about his guilt in a magazine article that was adapted for the Oscar-winning film "The Killing Fields."

In the Newsday letter, Koch disparages the rest of Schanberg's career this way: "He became metropolitan editor of The New York Times, one of the greatest jobs in journalism in the country. Then he was not. He was shunted to an op-ed column but remained in print in the premier paper of this country. Then he was not."

Schanberg's difficulties at The Times tell a great deal about his adopted role as a crusader. Schanberg says he resigned as metro editor in 1981 after four years of clashes with Rosenthal over his efforts to increase coverage of the poor, the homeless, rapacious developers and other issues not embraced by the paper's hierarchy.

"I wanted to cover the city in a way he did not," Schanberg says. "It was an arm-wrestling match every day."

Rosenthal called this "simply not true. None of us were happy with Sydney's work as metropolitan editor. He was not a very good metropolitan editor ... He was not leading the staff properly." Rosenthal says he helped arrange the op-ed column as a way of gracefully moving Schanberg out of the job, and that he doesn't have "the faintest recollection" of Schanberg first offering to resign.

As a Times columnist, Schanberg trumpeted the same themes about the downtrodden, even criticizing the city's newspapers -- and by implication his own -- for not adequately covering certain subjects. When his Times column was abruptly killed, Schanberg says he was given the same one-paragraph explanation that the paper made public.

"They simply were not going to tell me not to write about those things because they knew that would rebound negatively," Schanberg says. "They didn't want to say out loud, 'We don't want someone covering the poor.' "

Schanberg says Times editors tried to persuade him to remain with the paper, but that he told them: "You just stifled a voice on the op-ed page, a page devoted to diversity of opinion. You've got a freedom of expression problem here. You have to give me my voice back."

Instead, Schanberg took his typewriter across town to Newsday's upstart New York edition, where Koch became an increasingly regular target. He churned out such columns as "The Mayor Should Go Stand in the Corner," "What's Loony Is How Koch Handles the Homeless," "The Mayor's Special Baloney Is Rated Grade A" and "Edward Koch: The Mayor Who Knew Nothing."

One column described Koch as a "determinedly ignorant" man who seems "unable to resist administering the gratuitous insult or the kick in the groin."

Koch is not the type to suffer such kicks quietly; he is a master at using the media to fight back. At times he appears to be everywhere.

Here is Koch on "Face the Nation," calling the American Civil Liberties Union "nuts." There he is on "The McLaughlin Group," calling developer Donald Trump "a lightweight." He is even becoming a tastemaker, having just signed with WWOR-TV to do "Ed Koch's New York," in which he will review movies, plays and restaurants.

Last year, when Schanberg berated Koch for telling a group of Soviet schoolchildren that their government was "the pits," Koch fired back with a letter accusing the columnist of a "double standard," saying that he tolerates repression in leftist regimes while denouncing it on the right. (Schanberg calls this "a crock.") When Newsday insisted that Koch delete the paragraph because it looked like redbaiting, the mayor pronounced himself "shocked" and published it in "The Koch Column" in the Staten Island Advance.

Handing a reporter a batch of Schanberg columns -- with the most offensive passages highlighted in yellow -- Koch says that Schanberg "made a conscious decision to be a populist, because that's how you get lionized in this town. It's a little group {of journalists} that lionize one another. And I'm the No. 1 guy in this town to beat up on."

"I don't feel defensive about the column," replies Schanberg, who began as a $49-a-week Times copy boy in 1959. "I'm prepared to be judged. I put my name on it. I don't think I ever pick on anyone smaller than myself."