"I want the Japanese people to believe that there is hope for tomorrow", she said at a televised press conference. Though he has not announced a date for elections, reports say polls will be held on October 22.
Mr Abe said yesterday that he would dissolve Parliament on Thursday to pave the way for an election more than a year before it is due.
The decision is largely seen as aimed at taking advantage of Mr Abe's recently improved support ratings and opposition disarray.
On Monday, just hours before Abe's election announcement, Tokyo Governor Koike said she would lead a new conservative, reform-minded "Party of Hope" to offer voters an alternative to the LDP. His three-year term as party leader ends next September, and he will have to fend off any challengers from within the LDP to remain prime minister.
Other government sources have told Reuters Abe will pledge to use some of the revenue from a scheduled sales tax hike in 2019 to fund spending on education and child care.More news: Taco Bell's expansion plan includes 350 new restaurants that would serve alcohol
More news: Elon Musk just made a SpaceX FAIL video
More news: Volkswagen Group models to be electrified by 2025
As Mr Abe addressed reporters, more than 100 protesters gathered outside the Prime Minister's Office to demand his resignation.
Despite a recent run of growth, the election victor will also have to contend with a sluggish economy, as the heavily indebted country grapples with a low birth rate and a shrinking labour force.
A scandal surrounding the government approval process for a school building project sent support ratings for Abe's Cabinet plunging in July.
Another poll, from Kyodo News, put the percentage of undecided voters at 42.2%, which for some observers meant that bringing elections forwards by a year was not without risks for Abe.
Abe is expected to announce the election at 6 p.m. briefing in Tokyo. The increase in Abe's ratings may help him retain his coalition's two-thirds majority in the lower house of parliament. It has set a comparatively low bar of securing a simple majority of at least 233 of the 465 seats at stake.