George Clooney, 50, was taking part in a protest to warn of a humanitarian crisis in the volatile border area between Sudan and South Sudan.
The actor was detained alongside his father, Nick Clooney, but both have now been released after paying bail of $100.
George Clooney is a keen Sudan activist and has visited the area several times.
South Sudan celebrated its independence from Sudan in 2011, but relations between the two neighbors have worsened since then.
The country is one of the world’s poorest regions and has hardly any roads, railways, schools or clinics as a result of two decades of conflict leading up to independence from Sudan in July 2011.
Bitter disagreements remain over oil resources and borders, with conflict raging in the border region – the focus for George Clooney’s concern.
Speaking to reporters following his release, George Clooney said his key concern was the fate of those in the region.
“Best estimate is tens of thousands of people are going to die from starvation… this isn’t a famine, this is a man-made tragedy by the government of Khartoum to get these people to leave.”
“You never know if you are accomplishing anything… We hope it helps,” George Clooney said.
The actor said the arrest was his first, but added: “Let’s hope it’s my last.”
He was released three hours later after paying the bail fee.
Secret Service spokesman George Oglivie explained how the arrest unfolded: “George Clooney was arrested for crossing a police line at the Sudan embassy and he’ll be transported to the Metropolitan police department second district.”
Also arrested, said George Oglivie, were Martin Luther King III, son of the civil rights leader; Massachusetts Democratic Congressman Jim McGovern; Virginia Democratic Congressman Jim Moran; and National Association for the Advancement of Colored People President Ben Jealous.
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