Three weeks ago Fire Chief Charles Rule announced that Alexandria would require new firefighters to pledge not to smoke on or off duty, and threatened to fire anyone who broke the pledge.

Now he is famous.

Rule was interviewed yesterday for the CBS Evening News with Walter Cronkite. On Wednesday he will be the special guest of Secretary of Health Education and Welfare-Joseph Califano as the federal government kicks off a no-smoking campaign. And every day Rule receives more letters or telephone calls from all over the country about the issue.

"Personally, I just think this thing has been blown far out of proportion." Chief Rule said yesterday in yet another interview in his office. He said he did not understand why there had been a "groundswell" of interest in the media about the issue.

But the decision to forbid new fire fighters to smoke came at a time when the federal government is preparing to launch a major antismoking campaign and restaurants airplanes, and other public places are restricting smokers to special areas.

Laura Miller, a special assistant to HEW Secretary Califano, said her boss had read stories about the smoking ban in the newspapers and had decided to invite Chief Rule to the ceremonies that will open the antismoking drive set the Shoreham Hotel on Wednesday. Califano will introduce Chief Rule at the event, Miller said.

Chief Rule, 41, who has served as Alexandria's fire chief since May, 1976, says his new fame has had no effect on his personal life. Thus far he has been interviewed by, among others, The New York Times, The Christian Science Monitor, a number of television stations in the Midwest, and CBS News. Yesterday the fire chief had on his desk a request from National Public Radio.

Then there are the letters. One writer predicted that in 10 or 20 years Chief Rule "will be remembered as a visionary man, who helped us all to face facts, instead of living in a dream world."

Not everyone has been so friendly, however. The fire chief said he had received several anonymous telephone calls telling him the caller was a lawyer and that he would be taken to court. There have been no threats against him or his family personally, however.

A pipe smoker for 23 years until he quit last July, Chief Rule said he does not want people to think of him as a zealot who is out to stop the world from smoking cigarettes.

The fire chief said he decided not to hire fire fighters who smoke because under Virginia's Heart and Lung Act fireman who suffer from pulminary diseases or hear attacks are presumed to be incapacitated as a result of their job. They can retire, therefore, with an extra large pension. The fire chief says he does not believe the city should be subsidizing smokers.