Manning is accused of providing huge quantities of classified military documents to Internet whistle-blower WikiLeaks.
The protesters staged a sit-in in the middle of a highway after police refused to let them place flowers at a memorial statue honoring the Marines raising the flag at Iwo Jima in World War II.
"They wouldn't even let us get up to the memorial," said Ann Wright, a former Army colonel and State Department official. "It was disrespectful."
She, Ellsberg and 28 others were carted off by heavily armed riot police one-by-one, the Post said.
"I thought we had to do this, to show we had some fortitude," said Ann Wilcox, an attorney for the demonstrators. "If we had quietly gone back, we wouldn't have made the statement that we made."
Manning, 23, faces two dozen charges, including "aiding the enemy," which could carry a death sentence if he is convicted, the Post said.
Ellsberg, 72, said he has supported Manning since his arrest.
"I identify with him more than anyone else I've seen in the last 40 years," Ellsberg said.
While many passing motorists honked in support of the protesters, some disapproved, yelling "Traitors!" Lisa Cantoni, 43, of Woodbridge, Va., told the Post she believed the protests were wrong.
"We are Americans, and he's supposed to be on our side," she said of Manning. "He should be punished. He should not be freed. . . . That's preposterous."