Robert Stolarik for The New York Times

Robert S. Halper, a retired Wall Street trader, spends time each day in Zuccotti Park talking to protesters about politics and their thoughts on reforming the banking system.

But Mr. Halper, a 52-year-old Brooklyn native, never reveals two facts about himself: he is a former vice chairman of the New York Mercantile Exchange and the largest single donor to the nonprofit magazine that ignited the Occupy Wall Street movement.

“The whole thing is very surreal to me — the fact that I spent my whole career right across the street,” he said in an interview last week on a marble bench near the park. “It makes me a little anxious, to tell you the truth. It could go anywhere. I just pray that it ends peaceful.”

Mr. Halper said he first heard about the plan for protests in June when he visited Kalle Lasn, the editor in chief of Adbusters, a Canadian anticorporate magazine, in Vancouver. Over a steak dinner, the two longtime friends discussed Mr. Lasn’s project, a plan to fill Wall Street with protesters as a way to galvanize anger on the political left into a revolutionary movement resembling the Arab Spring.

“I rolled my eyes,” he said. “I was more interested in talking about health care.”

But Mr. Halper, who lives on the Upper West Side, had long been a supporter of the magazine, donating by his estimate $50,000 to $75,000 over the last 20 years since he was first attracted by the magazine’s spoofs on corporate logos and advertisements. So he wrote a check for $20,000 and returned to his life in New York.

A month later, the magazine sent an e-mail blitz to 90,000 readers and advocates calling for the occupation of Wall Street and setting the date for the first protesters to camp in downtown Manhattan.

“We sparked it,” said Mr. Lasn, 69, but “what they’ve done up until now — with a leaderless movement that is all-inclusive — that’s given them a kind of mystique that has launched a national conversation.”

The text of that initial call can still be found on the magazine’s Web site, which has been filled with photos and videos from the Occupy Wall Street protests. Mr. Lasn said the magazine’s circulation, now roughly 120,000, had expanded in the weeks since protesters took over Zuccotti Park on the Adbusters-selected date, Sept. 17.

The protest quickly grew beyond the influence of the magazine’s followers, as independent calls for similar protests in other cities began proliferating. The leaderless movement soon gained a measure of cohesion online from another unlikely place: Nebraska.

Two days after the start of the Occupy Wall Street protests, a pair of young Web designers, unable to travel to New York to take part in the demonstration, created a simple digital hub for the growing number of Facebook and Twitter pages dedicated to spreading the protest to new places.

“I thought, ‘Wow, it would be really great if there was a Web site that collected information about all of these,’ ” one of the designers, Ella, 25, said. (She spoke on the condition that she be referred to only by her first name to minimize the harassment she receives via e-mail.)

She and her fiancé threw together a page in “a few hours” and called it Occupy Together, and the design appears to have been emulated by others trying to organize in dozens of cities, including Boston, Seattle and Portland, Ore.

Mr. Halper, the Wall Street supporter of Adbusters, said he felt swept up in something larger but remained ambivalent about the protests. He does not claim any role in starting the movement, though he calls Zuccotti Park “the coolest place in New York.”

Mr. Halper was raised in East New York, Brooklyn, and Woodmere on Long Island, and his parents owned a liquor store on the Lower East Side. He began his career as a floor trader in 1983, swapping oil futures and living in Manhattan. Since retiring in 2007, he has focused on philanthropy, donating roughly $100,000 a year to a variety of causes, mostly related to health care and the arts.

He recently gave $2,500 to Mitt Romney’s campaign for president, after meeting him at a neighbor’s fund-raiser. “My giving is a little A.D.D. — like me,” he said, referring to what he described as his hyperactivity and wandering attention.

He readily admits to being a member of the so-called One Percent — the top slice of American earners, who have been vilified by the protesters. “The fact that I made a lot of money, things just worked out for me,” he said. “There’s some issues where we’re all in it together.”

Mr. Halper said his conversations with protesters had made him think a lot about what should be done. “If there’s pain, it should be shared,” he said. “The people who have money — they should pay something more, whether that’s in taxes or somewhere else.”