America Online’s plan to start charging for e-mails has run into the opposition of an array of not-for-profit organizations, who yesterday launched a campaign to persuade the Internet giant to change its mind.

More than 50 not-for-profit organizations joined together to try to convince AOL to abandon its plans to introduce a system under which some e-mail senders would pay to guarantee that their messages are delivered and not intercepted by spam filters.

“We’re vehemently opposed to AOL’s plans to impose an e-mail tax,” one of the leaders of the campaign, Timothy Karr, said in a conference call with reporters yesterday.

“It’s the first step onto a slippery slope that will dismantle many of the freedoms that Americans have come to know. AOL’s pay-to-send scheme threatens the free and open Internet as we know it,” said Mr. Karr, the campaign director for a group that advocates for alternative news outlets, Free Press.

Within the next month, AOL plans to roll out the service, which it has dubbed “certified e-mail.” Reputable companies and organizations could pay AOL and its partner, Goodmail, a penny or two per message for assured delivery. Customers receiving the special e-mails would see an icon confirming such messages as authentic and could open them without fear of being targeted for fraud or identity theft.

The coalition of critics said yesterday that the system will be the first big step towards a two-tier system for email service in which paid messages get priority and unpaid messages wind up in bulk e-mail folders or are never delivered at all.

“AOL guarantees over time that those using the free delivery system will suffer from a degradation of service,” said Gilles Frydman, a spokesman for the Association of Cancer Online Resources. He said his group sends 1.5 million emails a week about important research and treatment developments to cancer patients and their families.

The not-for-profit groups denouncing the new e-mail program said about 20% to 25% of their emails go to AOL subscribers. One of the founders of a large liberal advocacy organization, MoveOn.org, said paying the fees would cost thousands of dollars per week.

“Worst case scenario, we can probably afford the fees that AOL is levying,” said the Web activist, Eli Pariser. “The real tragedy is what it will do to the little guys who are just getting started.”

A spokesman for AOL, Nicholas Graham, charged that the critics had leveled a series of “phony charges” against the paid email program. He said the new system could not be used to deliver spam. He also ridiculed the notion that the pay-per-message scheme would disadvantage users of the traditional service.

“When the U.S. Postal Service introduced next-day delivery, did everyone abandon parcel post? Or First-Class stamps?” Mr. Graham asked. He said the warnings about an “e-mail tax” amounted to hysteria, since no portion of the fees involved would be assessed by the government. “One thing that allows people to get real excited is the word, ‘tax,'” he said.

The AOL spokesman insisted that the program would be free to e-mail recipients. “Consumers pay nothing. Zero. Nada,” Mr. Graham said. He did not address whether companies might recover the additional costs of the service by raising prices.

Mr. Graham said the American Red Cross has already signed up for the service and that other not-for-profits would be given a cut rate. “We made very clear in our negotiations that there would be a special level of service for non-profits,” he said.

However, Mr. Frydman said his cancer information group would not pay any fee, even a reduced one. “It’s just not acceptable. We are providing a free service,” he said with some exasperation.

Many of the groups campaigning against the pay-per-send system have a liberal outlook, including the Democratic National Committee and the AFLCIO. However, a couple of grassroots conservative organizations joined the protest, including the Gun Owners Action League and RightMarch.com.

“Anything that smacks of impairing our ability to communicate with our members is something that we don’t take lightly,” said Larry Pratt, executive director of the gun owners’ group.

He predicted that if AOL does not drop the paid e-mail plan, many members of his group would move their accounts elsewhere. “They’re already suspicious about government and corporations meddling in their private affairs,” Mr. Pratt said.

Most of the coalition members who joined a conference call with reporters stopped short of calling for a boycott of AOL, but Mr. Pratt said he would favor one “if AOL actually pulls the trigger.”

The group did not level criticism at other Internet providers who are involved with similar paid e-mail schemes. Yahoo has also partnered with Goodmail and is planning a test of certified e-mail to deliver bank statements and purchase receipts.

A division of Microsoft, Hotmail, uses a service called “bonded sender” to highlight legitimate e-mails. Companies participating in the program post a financial bond to certify that their messages are not spam.

AOL ceded no ground to the critics yesterday, insisting they will not succeed in derailing the program. “Never. Not going to happen,” Mr. Graham said in a statement. “Implementation of this timely and necessary safety and security measure for our members takes place in the next 30 days. Mark it on your calendars.”