Their causes include such diverse issues as global warming, gas prices and corporate greed -- though most seem to be fueled by the common thread of anger at the wealthy and powerful at the expense of the middle class and less fortunate.
The Occupy Wall Street website says organizers took their inspiration in part from the so-called Arab Spring demonstrations that have tried to bring democracy across the Arab world.
"We will be in a thousand cities in this country by the end of the month - hundreds of cities in other countries. We will see General Assemblies on six continents," read a post on the movement's website today.
But with the protests spreading to so many cities, there is no clear, single message, leaving many wondering what exactly people are protesting about.
"There is no one, unified message for the protesters, but that doesn't mean these protests are not real," said Jake Horowitz, the co-founder of PolicyMic, a news site focused on millennial politics. "From students protesting against tuition hikes to union leaders speaking out against the death of the middle class, people are angry and disillusioned with the economy and political process. They feel that a small few are racking up huge profits while the vast majority of Americans are suffering.
"More importantly," he added, "they feel that Washington is only representing the select few. These protests do not have a clear end-goal or aim in mind, but they represent a chance for people to vent their frustrations and commiserate, to finally raise their collective voices and get Washington to listen. While there is no one person or policy that is the target of these protests, this is not a fringe movement of hippies and radicals," Horowitz added.
Protests at Freedom Plaza in Washington, D.C. kicked off Thursday. A video on the Occupy D.C. website showed some protesters dressed up as wealthy investment bankers riding the subway, holding champagne glasses and toasting to the "1 percent" -- the wealthiest Americans.
Others chose to mark the 10th anniversary of the start of the U.S. war in Afghanistan at a church near Dupont Circle.
The Stop the Machine movement also joined the protest, although Occupy D.C. made clear on its website that the two groups are not affiliated.
The National Air and Space Museum was closed today after an estimated 100-200 protesters tried to enter with signs. When a security guard tried to stop them, he was held by the demonstrators, the AP reported. A second guard used pepper spray on at least one demonstrator.
In Georgia, several hundred people have joined the Occupy Wall Street movement, setting up their own demonstrations in Georgia with group names like the "99 Percenters" and Occupy Atlanta.
"The 99 percent of Americans who basically fall into the middle class or the lower class who are suffering from unemployment ... are suffering from corporate America basically gouging the middle class," protester Alethia Hyman said.
U.S. Rep. John Lewis, D-Ga., showed up at rally in Atlanta, hoping to lend a word of his support, but the crowds voted against hearing him speak.
Lewis shared what he had wanted to say with ABC: "I stand with you. I support you, what you're doing to humanize American corporations, humanize the American government and look out for those who have been left out and left behind."
In Los Angeles, protesters have been camping outside city hall for over a week.
Earlier this week, about 40 protesters from ReFund California, a coalition of advocacy groups and union members, barged past security guards into a California Bankers Association meeting at the Balboa Bay Club in Newport Beach, chanting, "Make banks pay!" the Los Angeles Times reported.
