Former Prime Minister Dr Mahathir Mohamad was in stable condition after his second heart bypass operation today, his daughter said.

Marina Mahathir told reporters that her 82-year-old father spent five hours on the operating table, and that the procedure went as planned.

"We feel very good, we feel very relieved," she said. "There was no big drama. I would like to thank all those who prayed for my father's recovery."

The hospital said in a statement that Mahathir had been transferred to the intensive care unit for observation, and would be kept sedated overnight.

"The next 48 hours is the crucial period in his post-operative care," the statement said. Family members were to spend the night at the hospital.

Mahathir, who ruled Malaysia for more than two decades, was admitted to Kuala Lumpur's National Heart Institute for elective surgery on Sunday after suffering two heart attacks in the past year.

He last appeared in public on Friday, waving the Malaysian flag as part of the country's celebrations marking 50 years since independence from Britain.

Mahathir led Malaysia for 22 years before handing power to the current premier Abdullah Ahmad Badawi in 2003.

Despite retiring from office, Mahathir remains an influential figure in Malaysian politics, and has become an outspoken critic of his successor, accusing him of nepotism and corruption.

The former prime minister was rushed to hospital in May after suffering a heart attack, his second in a year after a mild episode in November. Mahathir previously had bypass surgery in 1989.

Earlier, Mahathir's special officer Sufi Yusoff said the former premier was taken into the operating theatre at 9am. The procedure started at 9.30am.

The surgery team was led by cardiac surgeon Dr Yahya Awang.

Assisted by Mayo clinic expert

Bernama also had earlier quoted Marina - based on her entry in her blog RantingsbyMM - that Yahya's team would be assisted by Dr Hartzell Schaff, chief cardiac surgeon at the Mayo Clinic in the US.

"Dr Schaff has operated several times with Tan Sri Yahya at IJN before," Marina was quoted as saying.

Marina said that although it was decided during a December 2005 medical check up that Mahathir did not need a surgery, a quick succession of three minor attacks recently changed that outlook.

"Hence two weeks ago, he went in for some tests and an angiogram to determine the next course of action after which this decision to redo the bypass was made.

"At 82 (Mahathir will turn 82 on Dec 20), he is probably the oldest person to have a redo in Malaysia although Dr Schaff has done plenty in the US," she said.

Marina, writing in her blog, said her father was tough.

"God willing, all will be fine," she wrote. "But any prayers sent our way will be very much appreciated."