The leader of Japan's largest opposition party says it is a serious problem that the government decided to hold a state funeral for former Prime Minister Abe Shinzo without the involvement of Parliament.

Constitutional Democratic Party President Izumi Kenta said his party offers condolences on the death of Abe, who was gunned down during an election campaign speech. But he added that state funerals like the one held this time must not be allowed.

Izumi also said his party's leaders were not attending Tuesday's funeral.

Democratic Party for the People President Tamaki Yuichiro said he would attend the funeral and offer flowers for Abe.

He said it is regrettable that public opinion is divided over the state funeral, and that it is problematic that the government failed to sufficiently explain its decision to the public.

He also said there's an urgent need to create criteria for state funerals, and that his party will urge Prime Minister Kishida Fumio to do so.