BEIT-UL-MOQADDAS (Reuters) -- Israelis kept a nationwide vigil for Prime Minister Ariel Sharon on Saturday after he underwent emergency surgery to staunch renewed bleeding in his brain from a massive stroke.

Doctors at the hospital where Sharon lay in critical but stable condition said it could be days before they knew how much damage he had suffered. But medical experts believe that if he survives it is extremely unlikely he will ever return to work.

The death or incapacitation of Sharon, 77, who raised peace hopes by pulling Israeli settlers and troops out of Gaza in September to end 38 years of military rule, would create a huge vacuum in Israeli politics and the Middle East peace process.

At Jerusalem synagogues where the Jewish Sabbath was being observed and at Tel Aviv beach cafes where youths carried surfboards under their arms, Israelis anxiously awaited word on the ex-general's fate after a five-hour operation on Friday.

"We're all praying for him. He is a giant of our nation," said Jonathan Eilat, 47, wearing a prayer shawl at Jerusalem's Western Wall, the holiest site of Jewish worship.

Surgeons at Hadassah hospital staunched renewed bleeding in Sharon's brain on Friday but said they would only be able to assess the extent of damage in days ahead when he is weaned off drugs that they said were keeping him in an induced coma.

He was on a respirator after the operation, his third since being rushed to the hospital on Wednesday.

"There is always damage in cases like these. We have to wait to see how he will react when we lessen the dosage of the medications that are keeping him sedated," neurosurgeon Felix Umansky told reporters late on Friday.

Hadassah hospital director Shlomo Mor-Yosef said Sharon's brain scan following surgery showed "significant improvement". No further medical bulletins were expected, barring major developments, until after the Sabbath ends at sundown.

Nonetheless, throughout the Jewish state, radios were tuned to on-the-hour news broadcasts for any scrap of new information.

Sharon's doctors, speaking before Friday's surgery, cautioned against undue pessimism over his condition.

"For some reason, everyone is mentioning the less pleasant things ... any assessment is irresponsible -- things can go one way or the other," Shmuel Shapira, Hadassah's deputy director, told Israel's Channel Two television.

Sharon is reviled in the Arab world but increasingly seen by the West as having opened up new prospects for peace. He suffered his stroke at his Negev Desert ranch at a crucial juncture in Israeli politics, as he was fighting for re-election on a promise to end conflict with the Palestinians.

Underlining his pivotal role, U.S. Secretary of State Condoleezza Rice cancelled a trip to Indonesia and Australia because of concerns over his condition and the risk of another eruption of Middle East violence.

Political analysts said Israel's March 28 election, which Sharon had been widely expected to win as head of the new centrist Kadima party, would become an open race without him.

His deputy Ehud Olmert was named acting prime minister on Wednesday after Sharon fell ill.

Much of Sharon's popularity among Israelis stems from a belief he could take bold steps towards reconciliation with the Palestinians which others would not get away with, given his background as the archetypal hawk.

But two opinion polls published in newspapers on Friday suggested that under Olmert, Kadima would still win around 40 of parliament's 120 seats -- well ahead of Likud, the rightist party Sharon abandoned, and centre-left Labour.

Sharon had been campaigning on a platform of readiness to give up some occupied land in the West Bank, but has vowed to hold on to major West Bank settlement blocs, a prospect Palestinians say would deny them a viable state.

In Washington, White House spokesman Trent Duffy said Bush continued to pray for Sharon's recovery. Rice spoke to Olmert briefly by telephone and an Israeli political source said Egypt's President Hosni Mubarak also called to wish him well.

Palestinian President Mahmoud Abbas telephoned Olmert on Thursday to express hopes Sharon would recover, but some other Palestinians were less forgiving of the Israeli leader's harsh measures to fight a five-year-old uprising.

One preacher at Friday prayers in Gaza denounced him as "the man who wiped the smiles from the faces of children".

Even before Sharon fell ill, hopes for peacemaking were dwindling over a possible delay to Jan. 25 Palestinian elections and growing unrest in Gaza and the West Bank.