How many will show up, let alone stay to camp out to disrupt city centers for days, or months, to come, is anyone's guess. The hundreds at Manhattan's Zuccotti Park were calling for back-up on Friday, fearing imminent eviction. Rome expects tens of thousands at a national protest of more traditional stamp.
Few other police forces expect more than a few thousand to turn out on the day for what is billed as an exercise in social media-spread, Arab Spring-inspired, grassroots democracy with an emphasis on peaceful, homespun debate, as seen among Madrid's "indignados" in June or at the current Wall Street park sit-in.
Blogs and Facebook pages devoted to "October 15" - #O15 on Twitter - abound with exhortations to keep the peace, bring an open mind, a sleeping bag, food and warm clothing; in Britain, "Occupy London Stock Exchange" is at pains to stress it does not plan to actually, well, occupy the stock exchange.
That may turn off those with a taste for the kind of anarchic violence seen in London in August, at anti-capitalism protests of the past decade and at some rallies against spending cuts in Europe this year. But, as Karlin Younger of consultancy Control Risks said: "When there's a protest by an organization that's very grassroots, you can't be sure who will show up."
Concrete demands are few from those who proclaim "We are the 99 percent," other than a general sense that the other 1 percent - the "greedy and corrupt" rich, and especially banks - should pay more, and that elected governments are not listening.
"It's time for us to unite; it's time for them to listen; people of the world, rise up!" proclaims the Web site United for #GlobalChange. "We are not goods in the hands of politicians and bankers who do not represent us ... We will peacefully demonstrate, talk and organize until we make it happen."
By doing so peacefully, many hope for a wider political impact, by amplifying the chord their ideas strike with millions of voters in wealthy countries who feel ever more squeezed by the global financial crisis while the rich seem to get richer.
"ENOUGH IS ENOUGH"
"We have people from all walks of life joining us every day," said Spyro, one of those behind a Facebook page in London which has grown to have some 12,000 followers in a few weeks, enthused by Occupy Wall Street. Some 5,000 have posted that they will turn out, though even some activists expect fewer will.
Spyro, a 28-year-old graduate who has a well-paid job and did not want his family name published, summed up the main target of the global protests as "the financial system."
Angry at taxpayer bailouts of banks since crisis hit in 2008 and at big bonuses still paid to some who work in them while unemployment blights the lives of many young Britons, he said: "People all over the world, we are saying 'Enough is enough'."
What the remedy would be, Spyro said, was not for him to say but should emerge from public debate - a common theme for those camping out off Wall Street since mid-September, who have stirred up U.S. political debate and, a Reuters poll found, won sympathy from over a third of Americans.
A suggestions log posted at http://15october.net ("This space is ready for YOUR idea for the revolution") range from a mass cutting up of credit cards ("hit the banks where it counts") to "use technology to make education free."
For all such utopianism, the possibility that peaceful mass action, helped by new technologies, can bring real change has been reinforced by the success of Arab uprisings this year.
"I've been waiting for this protest for a long time, since 2008," said Daniel Schreiber, 28, an editor in Berlin. "I was always wondering why people aren't outraged and why nothing has happened and finally, three years later, it's happening."
Quite what is happening, though, is hard to say. The biggest turnouts are expected where local conditions are most acute.
Italian police are preparing for tens of thousands to march in Rome against austerity measures planned by the beleaguered government of Prime Minister Silvio Berlusconi.
Yet in crisis-ravaged Athens, where big protests have seen violence at times of late, a sense of fatigue and futility may limit numbers on Saturday. In Madrid, where thousands of young "indignados," or "angry ones," camped out for weeks, many also feel the movement has run out of steam since the summer.
Germans, where sympathy for southern Europe's debt troubles is patchy, the financial center of Frankfurt, and the European Central Bank in particular, is expected to be a focus of marches calling by the Spanish-inspired Real Democracy Now movement.
Complicating German sentiments, however, a series of small bombs found on trains has stirred memories of the left-wing guerrilla attacks that grew in the 1970s from frustration at a lack of change after the student protests of 1968.
CITY OF LONDON
British student protests a year ago were marked by some acts of violence by what authorities say were hard-core anarchists. Days of looting in London in August were put down to motives that mingled political discontent with criminal opportunism.
As an international center of finance, the City of London is key target. But organizers know strong police powers make setting up a Wall Street-style protest camp there far from easy.
"There's quite a bit of fatigue setting in," said one young veteran of last year's protests against higher university fees. "But if it's still going by Monday or Tuesday, I think that will excite students and they will head down. The City is much more the focus of people's anger now, compared to a year ago."
A long Saturday of rallies may start in New Zealand, where the Occupy Auckland Facebook page provides links recommending "suitable clothing ... a sleeping bag, a tent, food" -- but, in a family-friendly spirit, strictly no drugs or alcohol.
Asian authorities and businesses may have less to fear, since most of their economies are still growing strongly.
Tracking across the time zones, through towns large and small ("Occupy Norwich!" reads a website from the picturesque English city), the New York example has also prompted calls for similar occupations in dozens of U.S. cities from Saturday.
In Houston, protesters plan to tap into anger at big oil companies. As the world's day ends, hardy souls will be marching in Fairbanks. "We will be obeying traffic lights," insist the authors of OccupyAlaska.org, and they "will be dressed warm."
History suggests such actions are unlikely, of themselves, to change the world. As one anonymous poster at 15october.net writes, "Fleshing out ideas into living reality has always been the bugbear of radical politics." And while anger at corporate greed is widespread, there are plenty of voters who would agree with the Australian who posted on the OccupySydney site that those marching will be "the lazy, the paranoid, the confused."
But some analysts do see a potential for political change.
Jeff Madrick, a prominent economics writer, speaks warmly of the serious and reasonable debate he found at Zuccotti Park. Revolutions may be rare, but the protests could push lawmakers to act on some of the demands, he said last week: "It may begin to change public opinion enough to give Congress, people in Washington, the courage of their own convictions."
