Exit polls showed the 63-year-old maverick got the overwhelming mandate he sought from voters as his ruling Liberal Democrats (LDP) looked set to have a clear majority in government for the first time in 15 years.

Koizumi called the early election last month after rebels in his party shot down his long-held goal of privatising the gigantic post office, a move he has argued is the key to revamping the world's second-largest economy.

He had vowed to resign if the LDP did not win but the Tokyo Broadcasting Corporation, just moments after the voting closed, said its exit poll showed the LDP would get 307 seats in the 480-seat lower house of parliament.

They had only 249 seats when Koizumi dissolved parliament in a huff last month when defectors from the LDP blocked the privatisation of the Japan Post. And the LDP was defending 37 fewer seats as Koizum purged dissenters.

The main opposition Democratic Party was a distant second with 105, the projection said, giving Koizumi a renewed mandate to push through a reform he says can bring about major changes to Japanese politics.

Public broadcaster NHK said the LDP was expected to have won seats in a range between 285 and 325 while the Democratic Party was forecast to have won between 84 and 127 seats.

Official results were not expected for hours but senior LDP lawmaker Toranosuke Katayama expressed confidence.

"So far I have received good reports on the outcome," he told TV Asahi. "I believe voters supported our reform initiatives and our efforts to continue them."

Koizumi, one of US President George W. Bush's closest allies abroad, called the election more than two years ahead of schedule.

The early vote was seen as a risky gamble, with observers and even many in his party suggesting that he would drive the LDP, which has ruled Japan almost non-stop since 1955, out of power.

But within days of the decision last month, opinion polls showed that the wily Koizumi, who broke the mould of normally staid Japanese politicians, would muster strong popular support.