Opinion polls published on Monday showed Japan's main opposition party out in front, days after Prime Minister Yoshihiko Noda set the country on the path to a general election.

Twenty-five percent said they would vote for the Liberal Democratic Party in the December 16 ballot, while Noda's ruling Democratic Party of Japan was endorsed by 16 percent, a poll published by the Nikkei business daily showed.

Asked who would make the best prime minister, 37 percent said LDP leader Shinzo Abe, down three points from last month's poll, with Noda up seven points at 25 percent.

Noda's surprise move last week to call an election could be behind the bump in his popularity, the paper said.

Noda is Japan's sixth prime minister in as many years and the third since the DPJ came to power in 2009. He had been under pressure for months to call elections amid tensions with China, a slow post-tsunami recovery and a plodding economy.

Another poll by the liberal Asahi Shimbun newspaper said 22 percent of voters would choose the LDP in the proportional representation part of the ballot -- which accounts for 180 of the total 480 seats in the lower house -- against 15 percent for the DPJ.

Voters in all 300 constituencies will cast two ballots in next month's poll: one for a named candidate to be their parliamentary representative and one for a party.

These party votes are tallied across 11 regional blocs, with seats being awarded to each grouping according to the percentage of ballots they receive.

The latest polls reaffirm the view that while the LDP is faring better in voter support under Abe's recycled leadership, it is not likely to garner enough seats to govern alone.

Commentators expect some form of coalition after the vote, with narrowly-focused small parties possibly playing a disproportionately large role.

Two of those parties -- Japan Restoration Party and the Party of the Sun, led by two of Japan's most outspoken politicians -- have enjoyed intense local media coverage and have now united.

Acerbic former Tokyo governor Shintaro Ishihara and the feisty mayor of Osaka, Toru Hashimoto, agreed on Saturday to combine their parties in a bid to forge a "third force".

The Nikkei poll said 11 percent of voters support Hashimoto's Japan Restoration Party and four percent support Ishihara's Party of the Sun.

The Asahi poll showed combined voter support for the two parties at seven percent.

Both polls were conducted over the weekend.