Japan's ruling Liberal Democratic Party said on Tuesday that it would hold a slimmed-down election to pick a successor to Shinzo Abe, who is stepping down as both party chief and prime minister.

But a decision to exclude grassroots members from the vote has drawn a backlash from within the LDP.

The move, which means only the LDP's parliamentarians in the Diet and delegates from the party's local chapters can cast a ballot, makes Japan's top spokesman, Chief Cabinet Secretary Yoshihide Suga, a strong front-runner to replace Abe. The longest-serving Japanese prime minister announced on Friday that he was stepping down due to health reasons.

"We are in a time of emergency," said Shunichi Suzuki, head of the LDP's general council that handles the election process. "We must hold an election for forming the next government as soon as possible."

"If we hold a full-spec election, it would take two months to complete," Suzuki said, adding that the decision was unanimous.

But more than 140 lawmakers, including former defense minister Shigeru Ishiba, a popular candidate, signed a petition calling for a full vote ahead of the council meeting.

The LDP is expected to hold the vote on Sep 14, with an announcement on the date due on Wednesday. The party's leader will almost certainly become Japan's next prime minister because of the party's majority in the lower house of parliament.

Suga, 71, who has been Abe's right-hand man for nearly eight years, had reportedly won the backing of the largest faction of the LDP, and it is widely believed that he will stay the policy course set out by Abe.

Local media said Suga will formally declare his intention to run in the LDP leadership election on Wednesday, but until Tuesday, he preferred to be taciturn on his ambitions.

Besides Ishiba, Suga's most likely rival will be ex-foreign minister Fumio Kishida, who announced his candidacy on Tuesday.

Kishida, who was long seen as Abe's preferred successor but scores low in voter polls, emphasized his experience in economics and foreign policy, telling a news conference he would do his "utmost for the nation and the people".

Agencies contributed to this story.