Organizing a protest is fundamentally about logistics: where do people meet, how do they get on a bus, who will order portable toilets. Obviously, the Internet, like fax machines and copiers, has made the tasks easier. Before last weekend's protests, for example, people registered online for buses to New York. And a mass e-mail notice was sent out to New York protesters, informing them about public bathrooms in Midtown Manhattan and giving them a number to call in case of arrest.

But the Internet has become more than a mere organizing tool; it has changed protests in a more fundamental way, by allowing mobilization to emerge from free-wheeling amorphous groups, rather than top-down hierarchical ones.

In the 60's, the anti-Vietnam War movement grew gradually. "It took four and a half years to multiply the size of the Vietnam protests twentyfold," said Todd Gitlin, a sociology professor at Columbia University and longtime liberal activist.

The first nationwide antiwar march in 1965 attracted about 25,000 people. By 1969, the protests had grown to half a million. But increasing the numbers required weeks and months of planning, using snail mail, phone calls and fliers.

"This time the same thing has happened in six months," Mr. Gitlin said. Even though momentum behind the demonstrations didn't grow until a month ago, after Secretary of State Colin L. Powell's presentation to the United Nations (news - web sites), more than 800,000 people turned out in 150 rallies in the United States last weekend, from 100 in Davenport, Iowa, to an estimated 350,000 in New York City. In Europe, more than 1.5 million protested.

The protests had no single identified leader and no central headquarters. Social theorists have a name for these types of decentralized networks: heterarchies. In contrast to hierarchies, with top-down structures, heterarchies are made up of previously isolated groups that can connect to one another and coordinate.

Because no central decision-making authority exists, protests can be localized and can appeal to new groups and individuals who don't live in areas where social protest information would typically reach. For example, Mothers Acting Up was started two years ago by four women around a kitchen table in Boulder, Colo., a liberal college town. But with their Internet site, www.mothersactingup.org, they have been able to reach 600 like-minded members across the country, many of whom participated in marches last week.

Technology also spreads word of rallies to countries where free expression is limited. In Singapore, where the government does not allow demonstrations at the American Embassy, cellphone text messages went out, exhorting recipients to gather at the embassy anyway. The text messages, which work like mass e-mail messaging to mobile devices, attracted at least a half-dozen placard-carrying demonstrators at the gates at the appointed time. The police rounded them up for questioning.

"Whenever a new communications technology lowers the threshold for groups to act collectively, new kinds of institutions emerge," said Howard Rheingold, the author of "Smart Mobs: The Next Social Revolution," which documents self-organizing and leaderless movements. "We are seeing the combination of network communications and social networks."

His book tells the story of how cellphone text messaging helped bring down Joseph Estrada (news - web sites), the Philippine president who was ousted after protests in 2001 over corruption. Text messaging advertised instant rallies, encouraged people to protest by wearing black and provided updates on the impeachment trial.

(In the same way, cellphone messaging is potentially alarming for the Chinese government. Officials do not have centralized control over the network and therefore cannot censor it, the way they do the Internet.)

E-mail lists have allowed individuals to create groups that defy geography and time. Thousands of people have joined hundreds of antiwar lists, and diverse streams of messages fly back and forth quickly, vastly different from the information flow in hierarchies. Since the beginning of the year, 300 messages have been posted on a popular antiwar list in Sydney, Australia, that has almost 900 members. The notes range from solicitations for donations to United Nations updates to appeals for local volunteers.

This is mass mobilization, but also nimble mobilization. Protesting a war that hasn't begun requires a constant eye on the calendar of government action. And the movement's flexibility maximizes its impact, organizers say. A protest date can easily be moved, timed to affect the latest diplomatic maneuver.

"We are trying to stay a step ahead of the administration by our planning," said Damu Smith, chairman of Black Voices for Peace, one of hundreds of groups involved in last week's demonstrations. And staying ahead of the game "is absolutely strategically central in our ability to be effective in what we are doing."

Military theorists are fond of saying that future warfare will revolve around social and communication networks. Antiwar groups have found that this is true for their work as well.