United Way, the nation's largest charitable fundraising organization, was accused yesterday of becoming a "Charity OPEC" by spokesmen for rival fundraising groups that are seeking to break United Way's monopoly on lucrative corporate solicitation drives.

Spokesmen for several groups charged at a news conference that United Way officials have refused to let activist organizations, such as those representing women, blacks and environmentalists, join their drives and that these officials have blocked attempts by such groups to independently participate in corporate solicitation drives and contribution plans through payroll deductions.

"If you want to raise money from employes through payroll deduction, in most places you can't do it" without being a part of United Way, said Robert O. Bothwell, executive director of the National Committee for Responsive Philanthropy. The Washington-based group is a coalition of social action and volunteer groups that is leading the attack on the United Way.

"There's no reason for this monopoly," Bothwell said. "You can have one campaign that benefits many organizations, not just the United Way."

United Way officials attended the news conference at the International Inn here and held their own news conference immediately afterward at the same podium. They denied they were attempting to stop non-United Way groups from participating in corporate solicitation drives.

"We in the United Way are not mounting any campaign to keep anyone out of any corporation," said Rosendo Gutierrez, a Phoenix businessman and member of the United Way of America's board of governors.

But yesterday's news conference, which kicked off a weekend conference on employe fund-raising techniques sponsored by the philanthropy committee, is the latest salvo to be fired in a bitter conflict that has been dubbed the "Charity War."

At stake is the system of workplace solicitation and payroll deduction pioneered by United Way in 1949. It is considered the cheapest and most effective method of fundraising in the multibillion-dollar-a-year charity business.

In the face of Reagan administration budget cuts and changes in the tax laws, private donations from corporations and foundations are expected to drop sharply. However, donations from individual employes through payroll deduction plans are seen as potential growth areas. Last year, according to the philanthropy committee, American workers donated $1 billion to charities.

Gutierrez said the United Way traditionally has not seen itself as an activist organization lobbying for political causes. "We are only attempting to serve the health and human services" needs of individual communities, he said.

However, Gutierrez said, many of the groups affiliated with the philanthropy coalition espouse specific political causes. Among the groups in the coalition are The Gray Panthers, the National Organization for Women's Legal Defense and Education Fund, the National Black United Fund and Consumer Federation of America.

Oral Suer, a top official for the Washington-area United Way, said in a telephone interview that each local United Way decides who its recipients will be and and how the money is divided up. "We're totally reflective of the kind of community we live in," Suer said.

"In San Francisco," he said, "gay agencies receive support from the United Way. That does not mean to say that every single United Way in the country ought to support gay agencies."

He said the United Black Fund here headed by Calvin Rolark, the husband of City Council member Wilhelmina Rolark (D-Ward 8), has participated in the United Way drive for about 10 years. The United Black Fund is not affiliated with the National Black United Fund.

But philanthrophy committee officials said that in most communities white business leaders are "really calling the shots in United Ways."

"The United Way has become a Charity OPEC; it's a cartel," said Robert McGarrah, an assistant general counsel for the American Federation of State, County and Muncipal Employees. "We think the monopoly United Way does have . . . ought to be broken."