Labwani was arrested in Damascus earlier this week while returning to Syria from a trip abroad that included a meeting at the White House.
"We are deeply disturbed by reports that Dr. Kamal Labwani was arrested by Syrian authorities upon his arrival in Damascus ...," spokesman Scott McClellan said in a written statement. "We stress that the United States stands with the Syrian people in their desire for freedom and democracy.
"The Syrian government must cease its harassment of Syrians peacefully seeking to bring democratic reform to their country."
The White House call for Labwani's "unconditional release" comes amid building pressure on Syria to co-operate with the U.N. investigation into the murder of former Lebanese Prime Minister Rafiq Hariri last February in Beirut.
A defiant Syrian President Bashar Assad in a rare nationally televised speech Thursday from Damascus University said the investigation was a ruse designed to bring the nation to its knees, according to local media reports.
Labwani was a leading advocate of reform during the so-called, and short-lived, Damascus Spring that followed the death of strongman Hafez Assad and the assumption of his brother. He was imprisoned for three years beginning in 2001.
Upon release, he formed the Liberal Democratic Union, a political organization. It was not called a party, since political parties are forbidden.
The arrest of Labwani further increases tensions between Washington and Damascus, which Washington suspects of allowing, or not stopping, Iraq insurgents from using Syria as a transit point