The hearing in Richmond, Va., centers on Jose Padilla, a former Chicago gang member who converted to Islam. He was arrested at O'Hare International Airport in May 2002, then designated an "enemy combatant" by President Bush and sent to a naval brig in South Carolina.
Attorneys for Padilla and various civil rights groups say such detention of a U.S. citizen is illegal.
"Once you open the door to a power like that, where does it end?" Andrew Patel, one of Padilla's attorneys, asked the Washington Post. "If we believe someone has done something bad, we take them to court and prove it. It's a grade-school civics thing."
The government contends Padilla trained at al-Qaida camps and was planning to blow up apartment buildings in the United States.