Japanese citizens are casting ballots on Sunday in an upper house election, where Prime Minister Shinzo Abe's ruling bloc is expected to keep the majority.

Voting began across the country at 7 a.m. with the outcome expected to come shortly after the polls close at 8 p.m.

Sunday's vote is for half the seats in the less powerful House of Councilors of Japan's two chambers. Japan's upper house renews half of its seats every three years as its members serve six-year terms.

Media polls have indicated that Abe's ruling Liberal Democratic Party's coalition is on track to win more than half of the 124 seats up for grabs, but attention is being drawn on whether the ruling bloc will secure the two thirds' majority needed to amend the country's pacifist constitution.

For a constitutional revision, Abe needs a two-thirds majority in both houses of parliament and a simple majority in a national referendum. The ruling coalition currently controls a two-thirds majority in the lower house.