Sharon's response "was a very important sign" and indicated that his brain stem was working, said Dr Felix Umansky, the chief neurosurgeon treating Sharon.
However, he said it was still too early to assess what impact the massive bleeding he suffered on the right part of his brain would have on his cognitive abilities or on the left side of his body.
"We are just at the beginning of a very long way," Umansky said on Monday, adding that it will take several days to make a fuller evaluation.
"Once he talks to us ... and there are no other infections I will be willing to say that he is completely out of danger."
Sharon remained in critical condition and on sedatives, though at a lower level than before.
Umansky said they would continue reducing Sharon's sedatives over the next few days in hopes that he will have a greater response to stimulation, including opening his eyes.
As the sedatives levels dropped, Sharon began breathing on his own, though he remained connected to the respirator, Hadassah Hospital director Dr Shlomo Mor-Yosef said.
Doctors performed several pain tests on Sharon throughout the day and Sharon slightly lifted his right hand and leg and his blood pressure rose, Mor-Yosef said.
"It was a slight, but significant, movement," he said.
Sharon's ability to breathe independently means that the respiratory centre of his brain is working, but it does not indicate activity in other parts of his brain or even whether he will survive, experts said.
Mor-Yosef said Sharon's hand
movement was significant
Some doctors said that since Sharon suffered most of the damage to the right side of his brain, he has a greater chance of regaining his speech and comprehension, which are controlled by the left side.
One of Sharon's neurosurgeons has cautioned that it was unlikely he could function as prime minister again, but Umansky said it was too early to make such a determination.
Doctors kept Sharon in a medically induced coma and on a respirator since Thursday to give him time to heal from the trauma of the stroke and three brain surgeries. Umansky said the doctors could put Sharon under again if his condition worsens.
After doctors make a final assessment on the damage Sharon suffered, they were to alert Attorney General Meni Mazuz, who will then decide whether to declare the prime minister permanently incapacitated.
"The minute we know what damage has occurred, we will talk," Justice Ministry spokesman Yaakov Galanti said.
Acting prime minister
Since an acting prime minister is in place, there is no urgency to such a declaration, Galanti added. Ehud Olmert, Sharon's deputy, was named acting prime minister after Sharon suffered the stroke last Wednesday, and can serve in that role for 100 days.
In the event the attorney general declares permanent incapacitation, the Cabinet would have to elect a new prime minister within 24 hours, from among the five sitting Cabinet ministers from Sharon's Kadima Party who are also lawmakers, Galanti said.
"We are just at the beginning of a very long way"
Dr Felix Umansky, Chief neurosurgeon treating Sharon.
That group includes Olmert, a potential political heir. The 77-year-old Sharon, Israel's most popular politician, was seen by many here as the best hope for resolving the Israel-Palestinian conflict.
His abrupt illness and expected departure from the Mideast political stage has raised concern that momentum on territorial concessions, created by his recent Gaza Strip withdrawal, would be stopped, and that Sharon's successor wouldn't have the stature to forge ahead on drawing Israel's final borders.
Before his collapse, Sharon appeared headed to a landslide victory in March 28 elections at the head of the Kadima Party, which seeks further pullbacks while strengthening Israel's hold over major settlement blocs.
Olmert told the Cabinet on Sunday that he would work to carry on Sharon's political legacy.
Sharon suffered a severe stroke Wednesday, two weeks after a first, mild stroke, and was rushed to Hadassah Hospital.
Sharon's condition and the uncertainty it generated has unsettled Israelis, who have been anxiously following news reports for updates.
At the entrance to the hospital on Monday, three Jerusalemites hung up a white sheet with blue lettering in English and Hebrew that read, "Ariel Sharon, there is more to do, please wake up."
In the Gaza Strip, where Sharon is reviled for his tough policies on Palestinians, 40 masked gunmen from the al-Aqsa Martyrs' Brigades militant group held a demonstration against the Israeli prime minister.
Sharon was rushed to Hadassah
Hopsital after several strokes
One held a gun to a photo of Sharon that was labeled "the killer of children" and then burned the picture.
Doctors not involved in Sharon's care said that if he awakens, the extent of his responses could vary widely, from slight movements of the fingers or opening of the eyes, to a much fuller awakening.
They have also cautioned that there is no guarantee Sharon will wake up at all.
That Sharon can breathe on his own "tells us that one part of his brain is functioning, the respiratory centre," said Dr. John Martin, a professor of cardiovascular medicine at University College in London.
"It does not tell us how he is thinking, it doesn't tell us how he can speak, it doesn't tell us how he can move his arms and legs.
"His chances of survival are better than if the respiratory centre had been damaged, but that still does not mean he is going to survive. ... It is still highly probable that he will die," Martin added, noting that Sharon's weight and age work against him.