When reporters left the headquarters of the Chamber of Commerce of the United States yesterday morning, after a news conference in which its president claimed that business is "10-to-1 against" President Reagan's tax increase bill, a strange sight greeted them.

On the sidewalk was a publicity man from the National Association of Manufacturers with a handout claiming that "the vast majority of American business leaders" favor passage of the bill "as quickly as possible."

Steve Johnson, the NAM aide, said his unusual role as a sidewalk vendor was dictated by protocol. "I couldn't walk in on their press conference," he said, "but we're not prepared to accept their statement that business is opposed to this bill."

Reagan's decision to embrace and lobby for the Senate-passed bill that would boost taxes by $98.9 billion over three years has caused a major split in the business community, which had been remarkably united for 18 months behind his program of reducing taxes and curbing government regulations.

The split starts in the Chamber of Commerce, which is undergoing a painful series of public disputes between its professional staff, headed by president Richard L. Lesher, and its elected one-year chairman, Paul Thayer, the chief executive of LTV Inc., a Texas-based aerospace firm.

At the same time that Lesher was insisting that the chamber is solidly opposed to House passage of the tax increase bill, Thayer was repeating his claim, in a telegram to Reagan, that most members of the chamber's board of directors back the measure.

Things were tense on both sides. Milton E. Mitler, the chamber's director of media relations, told reporters that the staff had no knowledge of any poll of the 61-member board, which Thayer said had yielded the endorsement of the tax bill, and that, in any case, no such informal poll could change chamber policy.

Thayer was unavailable for comment, but his company's Washington spokesman, Julian Scheer, said the chairman had been angered to learn that on Thursday Lesher turned down a White House request that Vice President Bush and presidential counselor Edwin Meese III be allowed to use the chamber's television studio to tape messages supporting the tax bill for use by TV stations around the country.

"Childish," Scheer said. "Standard policy," Mitler replied. "The facilities cannot be used to oppose chamber policy."

Behind the chamber's civil war and the larger split in the business community is the agony of a powerful economic sector, which in 1981 signed up enthusiastically for Reaganomics and worked for its passage, but which now is wracked by uncertainty about whether -- and when -- it will work.

Then, the chamber, the NAM, the Business Roundtable (a group of major corporations), the American Business Conference (a group of smaller, high-growth firms) and scores of business associations and companies lobbied arm in arm with each other and the White House for tax changes that would encourage savings and investment, allow faster depreciation on facilities and equipment, and reduce the corporate tax burden to increase cash flow and profits.

But today, with swelling federal deficits and stubbornly high interest rates crimping some businesses and clobbering others, there is no unanimity on where the remedy should be found. Lesher was joined at yesterday's anti-tax-hike news conference by officials of the National Federation of Independent Businesses and the American Farm Bureau Federation, all of whom maintained that tax hikes would prolong the recession without cutting deficits or interest rates.

Lesher, saying the chamber's opposition was reaffirmed by a 9-to-3 vote of its executive committee earlier this week, said: "If you raise taxes, you just give the spenders in Congress license to spend more."

But the NAM, the Roundtable, the ABC and such interest-sensitive industries as the National Association of Home Builders and the National Association of Realtors have decided they can accept the prospect of somewhat higher taxes in hopes of getting the deficits down.

But some of them have footnoted their general support by saying that specific provisions of the legislation would have to be changed for them to endorse it.

And that is only the tip of the lobbying iceberg, for embedded in the bill are specific provisions that, in the words of Rep. Barber B. Conable Jr. (R-N.Y.), ranking minority member of the House conferees, "are matters of survival" for affected companies and industries.

"The lobbying is intense," Conable said, "because in trying to raise revenues without a general, across-the-board rate change, we have had to hit some sectors very hard."

Pharmaceutical and electronic firms that have taken advantage of Puerto Rico's tax shelters have been working furiously to protect them from change. The tobacco industry has been trying to cut back the proposed hike in cigarette taxes. Defense contractors, Conable said, are "up in arms" about proposed changes that would accelerate their tax payments. A fight over alternative methods of tax-leasing, which Conable has tried to compromise, has pitted industry against industry and, even within the autos and appliances, firm against firm.

"Any time you try to raise taxes," said Jack Albertine, president of the pro-tax-raise ABC, "it's very difficult to get people to support it."

But when the tax increase is backed by a "pro-business" president personally liked and supported by most business leaders, the twistings and turnings become really dazzling.