The body count has begun in what some of the combatants are already describing as "the last great newspaper war of the 20th century"--the battle between The Denver Post and the Rocky Mountain News.

Daily circulation and advertising revenues of the News, a morning paper owned by the Scripps-Howard chain of Cincinnati, have grown steadily since 1942, when it adopted a tabloid format.

Meanwhile, the Post, the self-proclaimed "Voice of the Rocky Mountain Empire," has echoed a national trend of decline among evening broadsheets.

Between 1970 and 1980, the population of the Denver metropolitan area increased by 30 percent. But daily circulation of the Post grew only 4.1 percent--and that was the good news. The bad news is that in a decade when an average of 800 people moved into the metro area every week, the Post added only 1.4 subscribers per week to its Sunday circulation--long the paper's greatest strength.

Last year the News' weekday circulation exceeded that of the Post for the first time. Weekdays, total paid circulation for the Post for the 12 months ended Sept. 30 averaged 266,260, while the News averaged 295,061.

Then the Los Angeles-based Times-Mirror Co. bought the Post, putting down $25 million in cash and agreeing to pay $70 million more over 20 years.

In the 10 months since new publisher Lee Guittar assumed operating control, the new owners have deployed an arsenal of conventional weapons to fight the News for supremacy.

The Post redesigned a stodgy format and softened the market with a barrage of advertising on billboards, radio and television. It disinterred the weekday business news from its burial ground at the back of the sports section, added new fashion, auto and sports sections, consolidated special features on a single page, and added splashes of color.

It also hired a gaggle of new journalists, including a new sports editor, a new editor for its Sunday magazine, Empire, and an award-winning photo editor.

The Post even lured away one of the News' top attractions, sportswriter Woody Paige, for $75,000 a year.

But its boldest move was the launching last September of a new morning edition, to compete directly with the morning News.

Guittar is also trimming costs, and Times-Mirror executives told securities analysts in New York last month the Post, which lost $1 million in 1980, has returned to profitability. Phillip Williams, Times-Mirror senior vice president for newspapers, said the Post is now "making a significant amount of money."

The new morning editions, available at first only on newsstands, "is going dynamite," according to news circulation director Howard Greenberg.

Ralph Looney, editor of the News, is unperturbed. "They haven't made even a blip on the chart," Looney said of the effect of the Post morning edition on circulation of the News.

The Times-Mirror Co. applied many of the same tricks of the trade when it acquired the Dallas Times-Herald in 1970, and Guittar was the editor in charge of the changes there as well. But although the Times-Herald spent a lot of money, the Dallas Morning News remains the dominant paper.

In December, the Post began home delivery of the morning edition in the suburbs, and Guittar hinted that the evening edition may eventually be scrapped entirely. "We've certainly got our eyes on what's happening in the world," he said. "Our goal is to produce the best newspaper in Denver. And I'd hate to have somebody not read it because it's not available."

Guittar recognizes there won't be an easy victory. "For the last 10 years," he said, "the News has been growing while the Post has been flattening out. We're just getting into the game. The Post has been out of it for a long time. But a year from now, we expect circulation to be up 20,000 to 25,000."

At the News, Looney has also beefed up his staff with reporters drawn from other papers in major cities, taken an inquisitive approach to city hall, and expanded suburban coverage to counter the Post's new emphasis on local news.

But so far, the News seems content to rely on the tactics that allowed it to sneak past the long-slumbering Post: comics, features and columns, specialized pull-out sections each day of the week and assignment of as many as five staffers to cover Denver Bronco football games, always the lead story on page one every Monday.

Any comparison between the Post and the News invites nasty comment from employes about the parent chains.

"Scripps-Howard has a well-deserved reputation for penny-pinching," a former News employe said. "It's a chain run by accountants rather than news people."

"Comparing Scripps to Times-Mirror is like comparing a discount chain to Nieman-Marcus. They both stay in business, but they reach different customers with very different products."

Although the "war" between the Post and the News has generated a more aggressive reporting style in both papers, some of the changes have prompted cynical remarks on the part of both staffs.

"The reader will be the biggest casualty," one reporter said, "if this circulation war degenerates into what Mark Twain once described as 'a battle of wits between unarmed men.' "