Japanese voters went to the polls on Sunday for an upper house election in which the ruling Liberal Democratic Party (LDP) may receive a surge of support after the assassination of former prime minister Shinzo Abe, a leading statesman and party power broker.

Abe, Japan's longest-serving modern leader, was gunned down on Friday during a speech in support of a local candidate in the western city of Nara – a killing the political establishment condemned as an attack on democracy itself.

Elections for seats in parliament's less powerful upper house are typically seen as a referendum on the sitting government, and the latest opinion polls already pointed to a strong showing for the ruling bloc led by Prime Minister Fumio Kishida – an Abe protege.

As the nation mourns, both the LDP and its junior coalition partner Komeito could gain from a potential wave of sympathy votes, political analysts said.

"The ruling LDP-Komeito coalition was already on course for a solid victory," James Brady of the Teneo consultancy said in a note. "A wave of sympathy votes now could boost the margin of victory."

Campaigning was halted on Friday after Abe's killing, but politicians resumed pre-election activities on Saturday.

There was an increased police presence when Kishida appeared at a campaign event in a city southwest of Tokyo and a metal detection scanner was installed at the venue – an unusual security measure in Japan.

Polls opened at 7am (2200 GMT on Saturday) and close at 8pm (1100 GMT). Media said 15.3% of voters had cast absentee ballots in advance.

A strong showing at the polls could help Kishida consolidate his rule, giving the former banker from Hiroshima a chance to carry out his goal of boosting defence spending.

It might also allow him to revise Japan's pacifist constitution – something even the hawkish Abe was never able to achieve.

"In the months ahead, the government is certain to seek to strengthen domestic security," Brady said.

"By undermining the public's general sense of safety and order, the event could also add further momentum to those key Abe causes like defence build-up and constitutional revision," he added.

Party power vacuum

Polls last week showed the LDP winning at least 60 of the 125 seats being contested on Sunday, compared with the 55 it now holds, allowing it to maintain the majority in the chamber that it holds with Komeito.

Reaching 69 seats in the upper house would give the LDP a majority, a threshold that had been seen as a stretch prior to Abe's killing.

Kishida, once on the more dovish side of the LDP, has shifted towards the right and said parts of the constitution may have elements that "are outdated and lacking".

Opinion polls show a majority of voters favour greater military strength.

But even a strong LDP performance will be overshadowed by the murder of Abe, who as leader of its largest faction still wielded considerable strength over policy and personnel decisions.

His death raises the spectre of a power vacuum and potential turmoil within the party, analysts said.

The small, populist Japan Innovation Party, which gained seats in a general election last year, could siphon off votes from the LDP. But since the party also backs constitutional revision, any advances it makes would be likely to bolster the LDP's reform goals.