Nobel Peace Prize winner and Holocaust survivor Elie Wiesel told the crowd that converged near the US Capitol that "Darfur deserves to live. We are its only hope."
Other speakers at the rally included Washington's Roman Catholic Archbishop Theodore McCarrick and actor George Clooney, who visited Darfur last week.
The march, mostly made up of white Americans, was a response to a call from 160 religious, political and humanitarian organisations.
There was no immediate estimate of the size of the crowd, but before the event organisers said they expected 10,000 to 15,000 people.
Smaller rallies were taking place in 18 other US cities, making it the largest public demonstration on Darfur since the conflict began three years ago.
'Show of solidarity'
"We know the march is not the beginning and end of it, but it's an improvement," said Rabbi Shawn Zevit, director of outreach for the Jewish Reconstructionist Federation.
"It is a show of solidarity, given the United States is one of few countries trying to exert pressure. There is momentum right now and the feeling we can influence things."
The secretary of state, Condoleezza Rice, said on ABC television that Washington was not getting enough support from other members of the UN Security Council to take more decisive action against Sudan.
Organisers were expecting
"We also do need more support, frankly, from members of the international community - from China, from Russia," she said.
The United States has called the violence in Darfur a genocide of the mainly African Muslim tribes by the government-backed militias known as Janjaweed.
George Bush on Thursday issued an executive order freezing the assets of four Sudanese deemed to have posed a threat to the peace process in Darfur.
But demonstrators called on the administration to do more.
Elamin Wadi, a refugee from Darfur who came to the United States in 2004, said: "We hope to send a message to the American government and then have the American government send a message to the Sudan government."
Public pressure on Sudan has been building in recent months, with several universities and some states divesting assets from companies doing business with Sudan.