Libyan rebels have control over much of Tripoli, though fighting between rival factions continued into Tuesday.
U.S. Sen. Kirsten Gillibrand, D-N.Y., in a statement, said with the rebel-backed TNC gaining momentum, it was time to bring the Lockerbie bomber to justice.
Megrahi, a former Libyan intelligence officer, is the only person convicted for the 1988 bombing of Pan Am Flight 103 in which 270 people were killed. He was released from prison by Scottish officials in August 2009 on compassionate grounds because of a terminal prostate cancer diagnosis. He was said to have about three months to live at the time.
Gillibrand said seeing images of Megrahi taking part in rallies supporting the regime of Moammar Gadhafi was a "slap in the face" to those committed to the fight against terrorism.
"The transitional government should immediately seek justice and hold this terrorist accountable by sending him back to prison," she said. "If we're ever going to win the fight against international terrorism, the rule of law must hold strong."
U.S. Sen. Robert Menendez, D-N.J., who led an inquiry last year into the Scottish decision to release Megrahi, said he felt there was a link to Megrahi's release and an oil deal between British energy company BP and the Libyan government.
A London inquiry determined there was no contact between BP and Scottish officials in the case. BP did lobby the British government but its involvement ended before Megrahi was sent back to Libya by Scottish authorities.