TOKYO, June 15, 2009 (AFP) - Japan's main opposition party won a closely-watched mayoral poll at the weekend, dealing a fresh blow to Prime Minister Taro Aso as he battles to win over voters ahead of a general election.

Aso, reeling from a cabinet aide's resignation last week and back-to-back mayoral election losses in May and June, has to call a vote by September, with opinion polls suggesting his Liberal Democratic Party is heading for defeat.

Sunday's vote in Chiba City, just east of Tokyo, came just two days after Internal Affairs and Communications Minister Kunio Hatoyama stepped down Friday over a row related to the privatisation of Japan's huge postal service.

Hatoyama was a long-time Aso ally who had led his election campaign in the ruling party's leadership race last September.

The latest election win for the main opposition Democratic Party had "deepened a sense of crisis" in the ruling coalition ahead of the general election, Jiji Press news agency said.

Pundits have said the upcoming general election will be the toughest fight yet for the ruling conservative party in its almost unbroken rule of Japan lasting more than half a century.

The latest opinion poll, conducted by the Mainichi daily over the weekend, showed voter support for the Aso government has sunk to 19 percent from 24 percent in mid-May.

The Democratic Party hailed Sunday's election win, in which its candidate Toshihito Kumagai, 31, also backed by the opposition Social Democratic Party, scored a landslide win.

The local election board said Kumagai beat former deputy mayor of Chiba, Kojiro Hayashi, 63, backed by the ruling Liberal Democratic Party and its junior coalition partner New Komeito, by 170,629 votes to 117,560.

"The Democratic Party will certainly topple the government in the general election to be held soon," Katsuya Okada, secretary-general of the resurgent party said in a statement after exit polls showed the party was set to win.

The new mayor, Kumagai, told supporters: "I believe this shows voters' will to change politics that have remained unchanged until now."

The opposition won mayoral elections in two other major cities in April and May. An election for the Tokyo metropolitan assembly is set for July.