TOKYO - Japan is planning to establish standards for when state funerals can be held, in a nod to the public fury that erupted over a service to send off former premier Shinzo Abe on Tuesday.

Prime Minister Fumio Kishida, who has vowed to look into issues such as costs as Cabinet approval plunged to new lows, said on Wednesday that it was "important to keep a record of the grounds on which Mr Abe's state funeral was held, so as to facilitate discussions".

Deputy Chief Cabinet Secretary Yoshihiko Isozaki said that neither the timeline nor the scope for the post-mortem has yet been set.

Issues related to state funerals are expected to be discussed during an extraordinary session of Parliament from Oct 3 to Dec 10.

Lawmakers from both the ruling Liberal Democratic Party (LDP) and opposition parties have said that it is necessary to set rules for holding state funerals.

The government said on Wednesday that the four-hour state funeral service for Mr Abe - held 81 days after he was assassinated on July 8 - was attended by 4,183 people, including Japanese politicians, businessmen and foreign luminaries.

Meanwhile, 25,889 members of the public left floral tributes for Mr Abe at stands set up near the Nippon Budokan arena where the memorial took place.

However, media polls have shown that at least six in 10 members of the public were against the state funeral. Largely peaceful protests broke out across Japan on Tuesday.

Leaving aside the fact that Mr Abe was a hugely polarising leader and nearly half of LDP lawmakers have links to the questionable Unification Church, the apparent lack of due process in the state funeral has added fuel to the fire.

Mr Kishida made the seemingly unilateral decision on July 14 to hold a state funeral, which was then signed off by the Cabinet on July 22.

Opponents say that the rubber-stamp decision ignores due processes, lacking the accountability that would have come with the checks and balances of a Diet debate.

The price tag, of at least 1.66 billion yen (S$16.5 million), also did not come under parliamentary scrutiny.

Furthermore, some legal experts have questioned the constitutionality of the event, given that state funerals fully funded by taxpayers' money are a relic of Japan's imperial past.

LDP secretary-general Hiroshige Seko suggested on Tuesday that the length of a PM's tenure could be one possible criterion for a state funeral. Mr Abe was Japan's longest-serving leader, having been in office for eight years and eight months - from 2006 to 2007, and 2012 to 2020.

The opposition Japan Innovation Party is reportedly tabling a Bill suggesting that approval of the Diet must first be obtained after considering factors such as rationale and cost estimates before a state funeral is held.

Mr Abe's send-off was only the second state funeral in post-war Japan, after a service in 1967 to honour Mr Shigeru Yoshida, who led from 1946 to 1947 and 1948 to 1954.

Mr Eisaku Sato, who was premier from 1964 to 1972 and is still the only Japanese PM to win a Nobel Peace Prize, was not given a state funeral when he died in 1975. Neither was Mr Yasuhiro Nakasone, who led from 1982 to 1987 and died in 2019.

Their memorial services were smaller in scale. Most were either private, or jointly held by the government and the LDP, with the costs split equally.

Mr Nakasone's funeral, for example, cost taxpayers 96 million yen - 5.8 per cent of the budget for Mr Abe's state funeral.