Megrahi, who was freed from prison in Scotland on compassionate grounds due to his health in 2009, was taken from his Tripoli home to a private hospital, his brother Abdulhakim told the Reuters news agency.
His health is said to have “deteriorated quickly” and received a blood transfusion as part of his treatment.
Megrahi is the only man convicted of the bombing of Pan Am Flight 103 as it flew from London to New York.
All 259 people on board the aircraft were killed in the bombing in 1988 and 11 others on the ground in Lockerbie, Dumfries and Galloway, were killed by falling wreckage.
Britain freed him in 2009 on compassionate grounds after he was diagnosed with advanced terminal prostate cancer and was given just three months to live. Three years later he is still alive.