Most of the demonstrators had planned to protest the meeting of the International Monetary Fund, originally scheduled for Saturday, but changed their focus after the attacks.
The Washington Post reported that 11 people were arrested and that pepper spray was briefly used during the day's events, which included protests by the International Action Center and the Anti-Capitalist Convergence.
The majority of protests were peaceful.
The IAC rally drew a large coalition of different groups from across the nation that held signs with sayings such as "No War, No Racism" and "The real terrorists work in the White House."
ACC rally participants were redirected to the IAC rally after being surrounded by police for more than an hour.
The combined group marched two miles down Pennsylvania Avenue toward the U.S. Capitol. The streets were lined with police in full riot gear.
Participants used spontaneous drum circles, theatrical demonstrations, large puppets and flag burnings to try to explain the terrorist attacks as products of U.S. foreign policy, including U.S. support of Israel and support of the Taliban during the Cold War.
Emilita Poling of New Haven, Conn., said she saw a direct link between U.S. actions overseas and the terrorist attacks. "We have sown violence overseas in Guatemala, Vietnam and Chile," she said. "We will continue to reap what we have sown."
Many protesters felt the United States should involve the world community in responding to the terrorist attacks.
"I think its a question of a crime rather than an act of war," said Heide Kober of Efland. "We should identify the criminals and bring them to justice on a global level."
The IAC rally included a three-hour-long series of speakers and moments of silence for the victims of terrorist attacks throughout the world.
Che Huffington of the Kensington Welfare Rights Union, an activist organization for poor and homeless people, advocated an alternative U.S. response. "War is not the answer," he said. "Let us bomb the world with food, housing and employment."
But some disagreed with the causes of the protesters.
Holding a sign stating, "Overthrow the Taliban," Michael Bright, a student at Johns Hopkins University, said he did not think the protesters were really serious about their causes.
"The protesters weren't making any sense," he said. "All the things they were saying and all of their signs were speaking in very vague terms. They're just people who want to protest something."
The State & National Editor can be reached at firstname.lastname@example.org.
To get the day's news and headlines in your inbox each morning, sign up for our email newsletters.