President Barack Obama has called Richard Holbrooke "a towering figure in American foreign policy" and said he is praying for the critically ill diplomat's recovery.
Mr Holbrooke, 69, the president's special envoy for Afghanistan and Pakistan, was described as being in stable, but critical condition after undergoing more than 20 hours of surgery for a torn aorta, the large artery that takes blood from the heart.
The veteran diplomat was meeting with the secretary of state Hillary Clinton at the State Department on Friday when he suddenly collapsed and was taken to George Washington University Hospital a few blocks away. Mr Holbrooke was seen walking under his own power to the department's parking garage with a person from State's medical office.
His family as well as Ms Clinton and Admiral Mike Mullen, the chairman of the Joint Chiefs of Staff, have been with Mr Holbrooke at the hospital. Ms Clinton visited on Friday night and again on Saturday.
"Richard Holbrooke is a towering figure in American foreign policy, a critical member of my Afghanistan and Pakistan team, and a tireless public servant who has won the admiration of the American people and people around the world," Mr Obama said in a statement.
He said he had spoken to Mr Holbrooke's wife, Kati, on Saturday "and told her that Michelle and I are praying for Richard".
"We continue to pray for his recovery, and support his family in this difficult time," said the president.
Hospital officials referred all questions about Mr Holbrooke's to the State Department.
A State Department spokesman P.J. Crowley said the surgery on Mr Holbrooke's torn aorta was completed on Saturday morning.
A torn aorta, or aortic dissection, is a condition in which a tear develops in the inner wall of the aorta allowing blood to enter the vessel wall and if not treated quickly can lead to rapid death. As blood enters the wall it reduces blood flow just as if there were a severely bleeding wound, leading to possibly serious complications in the brain and other areas of the body, according to medical experts.
Even if the surgery has stabilised Mr Holbrook's condition, recovery can be expected to take considerable time.