Tokyo will vote for a new governor this Sunday - for the fourth time since 2011.

A record 21 candidates are running to be leader of the Japanese capital, which is hosting the 2020 Olympics, although latest media polls show three candidates with a wide lead ahead of the rest.

Separate polls by Nikkei, Kyodo and Mainichi news agencies all show former defence and environment minister Yuriko Koike, 64, leading the pack. The former lawmaker with the ruling Liberal Democratic Party (LDP) is running as an independent after failing to win the party's backing.

She has a marginal lead ahead of her main competitors - former internal affairs and communications minister Hiroya Masuda, and former journalist Shuntaro Torigoe.

Top of the new governor's agenda will be to steer the city towards a successful Olympics and Paralympics in 2020.

Tokyo's preparations for the Games have been hit by a long list of snags, including a scrapped stadium design by the late architect Zaha Hadid, accusations of plagiarism over its official logo, and alleged bribes linked to the host bid.

The new governor will need to deliver a top-notch event while keeping a lid on ballooning costs: Japan's Olympic Organising Committee has asked Tokyo to shoulder part of the expenses.

The tight race has spurred people to vote in unprecedented numbers: Early voting figures from July 15 to 24 show some 461,000 people have cast their ballots - a 66 per cent increase over the corresponding period in the last election in 2014.

Sunday's winner will lead Japan's most populous city of 12.97 million people. Tokyo itself is among the 20 richest economies in the world, with a 2014 gross domestic product of 93 trillion yen (S$1.2 trillion).

Japan is divided into 47 prefectures, each with a local government that is largely autonomous though subject to national laws. Governors control metropolitan affairs.

Sunday's polls come in the wake of three resignations - the elected governor in 2011 quit a year later to run for a parliamentary seat, and his two successors stepped down in disgrace over money scandals.

The three leading candidates have pledged to solve the urgent problems dogging the metropolis - including more than 8,000 children on waiting lists for nursery schools, eldercare for a rapidly greying population and a yawning rich-poor divide.

But both the centre-right Yomiuri Shimbun and left-leaning Asahi Shimbun were united in their critique of the campaign, noting that candidates were still "prioritising slogans" and lacking in specific "road maps to the futures of the capital they envision".

Mr Masuda, who is backed by the LDP, has focused on his administrative experience as governor of sparsely populated Iwate prefecture, which has a low population density, from 1995 to 2007.

Said the 64-year-old: "I do not need any fancy slogans; I know what it takes to be governor, how to bring the different municipalities together to take Tokyo to greater heights."

Mr Torigoe, 76, said his experience as a reporter gave him the human touch and helped him "identify urgent problems from the lenses of a layperson, not a politician".

But his campaign was hit by allegations last week by a tabloid that he had made inappropriate advances to a young woman in the past. He has lodged a criminal complaint.

However, media polls suggest that Ms Koike could triumph, given her appeal to Tokyo's large numbers of unaffiliated voters because of her lack of backing from any specific political party and vast experience in politics.

"The image of her as a heroine who is being bullied in a male-dominated society may be prevalent," an assemblyman has said.

Ms Koike is well aware of this and has urged people to vote: "If independent voters don't go to the polls, that will mean death for me."