Mahathir is the country's longest-serving premier, having been in power for 22 years before retiring in late 2003. He still wields influence on politics and business and holds advisory roles for state oil company Petronas and car-maker Proton.
"He had a mild heart attack late last night, early this morning," son Mokhzani Mahathir told Reuters. "He is under observation, the condition is stable. The doctors gave some medication. He is likely to remain in hospital for a few days."
Mahathir was taken to Kuala Lumpur's National Heart Institute at about 4 a.m. (2000 GMT), an official there said. "We cannot give you any more details at this point," she added.

Copyright Reuters, 2006