SEOUL, Oct. 13 (Yonhap) -- Campaigning for the upcoming Seoul mayoral election officially kicked off Thursday, with rival candidates mounting offensives against each other over ethical qualifications and policy pledges to win the key election ahead of next year's major polls.

The Oct. 26 by-election pits Rep. Na Kyung-won of the ruling Grand National Party (GNP) against Park Won-soon, an opposition-backed candidate without party affiliation, with Na leading in recent polls by close margin.

The vote was set up after former Seoul Mayor Oh Se-hoon voluntarily stepped down in August after he failed to block an opposition-led free school lunch program in the city's first-ever referendum.

The ruling camp has launched an offensive against Park, a lawyer-turned-activist in his first run for an elected post, raising allegations that he evaded his military service and questioning his opaque NGO and pro-North Korean activities. Na, a two-term lawmaker, also sought to portray Park as a political novice with no proven administrative capabilities.

The 55-year-old Park, who founded a local charitable foundation, is facing criticism over receiving big donations from conglomerates that he had accused of engaging in unfair business practices. In previous debates, Park defended himself, saying the donation process was legitimate and the money was used for projects for less well-off people.

The ruling camp also raised suspicions that he was adopted by his grandfather's brother in violation of the law to avoid the mandatory military duty, a special favor given to families with one child.

Park, who vowed not to run a negative campaign, stressed that it is time for change, trying to galvanize voters by criticizing the debt-heavy municipal government led by Oh.

Park said he would adopt policies to help create more jobs based on his know-how from NGO activities and offer affordable rental houses for low-income families. Na used her image as a mother of two children, one with Down syndrome, promising to improve day care services for working moms.

With the official campaigning period kicking off Thursday, political heavyweights from each camp took to the streets and participated in major public events to drum up votes.

Among them is the GNP's leading presidential candidate Park Geun-hye, who received full attention after she vowed to hit the campaign trail on behalf of Na for the first time in nearly four years.

Park, who aspires to become South Korea's first female president, has so far taken a hands-off approach to party affairs amid soured relations with President Lee Myung-bak after she lost to Lee in the party's 2007 primary to select a presidential candidate.

Meanwhile, the opposition camp is eying active support by star

IT entrepreneur Ahn Cheol-soo, who was once considered as an unrivaled candidate for Seoul mayor. After Ahn dropped his bid in early September to endorse Park Won-soon, the lesser-known lawyer's single-digit approval rating had soared ahead of all other candidates.

Analysts say that the upcoming by-election would set up a preliminary stage not only for political parties but also major presidential contenders ahead of the April parliamentary elections and the December presidential race next year.