Many lawmakers in both parties oppose Obama's request for Congress to authorize using military force against Syria for a deadly Aug. 21 chemical gas attack the Obama administration blames on President Bashar Assad. Citing intelligence reports, the administration reported 1,429 people died, including 426 children.
The Senate Foreign Relations Committee cleared the authorization measure earlier this week and the first votes by the full Senate could come Wednesday.
Concerns over military action spawned other protests across the country, including one in New York City's Times Square and a prayer vigil in Boston that echoed Saturday's massive gathering at the Vatican.
In New York, anti-Wall Street activists joined the protest along with some Syrian expatriates who said they supported the Assad regime. Some protesters carried signs saying, "No more wars for corporate profit," and "Cut the Pentagon, not food stamps."
In Indianapolis, about 150 protesters clustered around the Indiana Statehouse in a church-organized protest opposing military intervention. Other protests were reported in Grand Rapids, Mich., Lincoln, Neb. and Los Angeles, as well as a prayer service in New Orleans.
Benjamin, who earlier this year interrupted Obama several times as he delivered a speech on national security, said a cross-section of Americans, many of whom disagree on a variety of issues, are united against military intervention.
"We have suddenly found ourselves united as Americans, overwhelmingly saying we will not let you drag us into another war," Benjamin shouted into a megaphone in front of the White House.
Benjamin said the public will get the chance to see if we have "democracy in action."
"The whole world is watching," Benjamin said.