Former Indonesian leader Suharto is suffering from multiple organ failure, doctors treating him have said.
Mr Suharto has always denied the allegations against him
He is losing consciousness and having "very bad breathing difficulties", the Jakarta hospital where he has been since last week said in a statement.
Family members of the 86-year-old have gathered to pray at his bedside, where he remains in a critical condition.
Indonesia's president has meanwhile cut short a visit to Malaysia because of the deterioration in Suharto's health.
The Indonesian embassy in Kuala Lumpur said Susilo Bambang Yudhoyono was flying home to Jakarta several hours earlier than planned following a three-day official visit.
His deputy, Vice-President Jusuf Kalla, visited the hospital where Suharto is being treated on Friday evening, but later left without speaking to the media.
Suharto had been in seriously ill for some time before he was admitted to Pertamina hospital a week ago with anaemia and low blood pressure.
After initially responding will to a blood transfusion and kidney dialysis, his condition deteriorated rapidly on Friday and by 1700 (1000 GMT) he had begun to lose consciousness and was having difficulty breathing, the hospital said.
RISE AND FALL OF SUHARTO
Born in Java, June 1921
Comes to power in 1965 after alleged Communist coup attempt
Formally replaces Sukarno as president in March 1967
Modernisation programmes in the 70s and 80s raise living standards
East Timor invaded in late 1975
Asian economic crisis of the 1990s hits Indonesian economy
Spiralling prices and discontent force him to resign in May 1998
Judges rule he is unfit to stand trial for corruption in 2000
Transparency International says he tops the world all-time corruption table in March 2004
Describing his health as "critical", doctors said the former president was placed on a ventilator after his brain and other organs failed when his blood pressure fell.
"He is still in an unstable condition," said Muhammad Munawar, a member of the medical team. "We cannot say how long," he added, when asked how long Suharto would survive.
The Indonesian Health Minister, Siti Fadilah Supari, said she was pessimistic and that the ventilator was a "last attempt" to keep him alive. "I don't think it will help," she said.
Relatives at his bedside have been praying and reciting verses from the Koran, family aides have told the Associated Press.
The hospital said on Saturday morning, however, that Suharto's condition had stabilised and that his vital signs had improved.
Suharto came to power in Indonesia in the aftermath of an abortive coup in 1965 and ruled the archipelago with an iron fist for the following three decades.
Many Indonesians want Suharto to face corruption charges
His supporters credit him with leading his country from poverty to relative prosperity, making Indonesia a force to be reckoned with in Asia.
But this economic growth came at a price - his regime was repressive and he repeatedly ignored demands for political reform. He was also accused of allowing human rights abuses, most notably in East Timor.
Since being overthrown in 1998 amid political upheaval triggered by the Asian financial crisis, Suharto has suffered many health problems, including several strokes.
A criminal case against him for corruption was blocked in 2006 after his lawyers argued that the 86-year-old was too ill to stand trial.
In August, judges began hearing a civil case brought by prosecutors seeking $440m (£224m) they claim disappeared from a state scholarship fund, and $1.1bn (£562m) in damages.