The Democratic Party of Japan pledged to stand firm against the government's security legislation in the Diet as it held its party convention in Tokyo on Sunday.

In adopting an action plan, the main opposition party also lashed out at "Abenomics," Prime Minister Shinzo Abe's economic program, claiming it is widening economic and social inequalities in the country.

The DPJ called for "cautious and sufficient" Diet debate on legislation aimed at giving the Self-Defense Forces bigger roles abroad. The government is expected to submit the legislation to the Diet in late April, after a nationwide series of critical local elections.

The DPJ vowed to defend Japan's principles of peace and its exclusively defense-oriented stance under the Constitution and draw people's attention to the "danger" of the Abe administration.

DPJ President Katsuya Okada, who was elected president in January, had to skip the convention to undergo retinal detachment surgery Thursday.

The party, which got its first taste of power from September 2009 to December 2012, pledged to rebuild itself and return to its status as a viable alternative to Abe's conservative ruling Liberal Democratic Party.

The DPJ, admitting to its "defeat" in the December's Lower House election, said it plans to expedite rebuilding work because the next Lower House poll may be held at the same time as the next Upper House poll, which is scheduled for summer 2016.