The LDP’s Tokyo chapter, led by economic revitalisation minister Nobuteru Ishihara, instead threw its weight behind former internal affairs and communications minister Hiroya Masuda, 64. Mr Ishihara apologised last night and said the election result was a “complete defeat”.

Koike yesterday vowed to transcend party affiliations in considering policies for Tokyo’s greater good. She told supporters: “I hope to be able to speak freely without the shackles of any party, so as to consider a wide range of policies for a new, better Tokyo.”

Political watcher Jeffrey Kingston, director of Asian Studies at Tokyo’s Temple University, told The Straits Times that the lack of LDP backing could have helped her, because “the last two governors anointed by the LDP had resigned in scandal”.

The election was billed as a three-horse race in a crowded field of 21 candidates. Ms Koike, Mr Masuda and the opposition-backed journalist Shuntaro Torigoe, 76, were seen as the leading contenders.

45.7% for Koike

28.5% for Hiroya Masuda

21.2% for Shuntaro Torigoe

But it was not even close.

National broadcaster NHK, which held its own exit polls, said Ms Koike’s win was “assured” within one minute of voting stations closing at 8pm local time (7pm Singapore time).

And with 83 per cent of the votes cast officially counted as of 11.25pm yesterday, Ms Koike had 45.7 per cent – ahead of Mr Masuda’s 28.5 per cent and Mr Torigoe’s 21.2 per cent.

Koike is not only Tokyo’s first female governor, but also only the seventh female governor across 47 prefectures in Japan.

Visibly moved, her voice cracked as she alluded to shattering a glass ceiling in a country that traditionally ranks low in gender equality indices.

She told her supporters: “I will firmly advance pro-women policies that can lead to a better, happier Tokyo.” Her top priority, on this front, is to ensure there are enough places in childcare centres. A shortage has led to over 8,000 children being placed on waiting lists in the capital.

She also pledged to address elderly care for Tokyo’s greying population, as well as work-life balance.

A governor’s term is supposed to last for four years. But Tokyo is already voting for its fourth governor since 2011, after the first quit to run for a parliamentary seat, and his two successors quit in ignominy over embarrassing money scandals.

Koike, who is a former translator and news anchor fluent in Arabic and English, now leads a sprawling metropolis the size of three Singapore islands with a population of 12.97 million citizens. Tokyo has an annual budget approximately the size of Sweden’s, and an economy the size of Indonesia’s.

Koike will lead the city in preparing for the 2020 Olympic and Paralympic Games. One of her first duties will be to travel to Rio de Janeiro for the Aug 21 closing ceremony of this year’s Olympic Games, to accept the flag as the next host.

Tokyo’s preparations for the Games have been hit by a long list of snags, including ballooning costs. Prof Kingston said: “Her main job as Tokyo governor is to rescue the Olympics from the constant cascade of scandals that have really damaged the reputation of Tokyo’s winning the host bid.”

Yesterday, Koike pledged to be more transparent in how the money is being spent.

SIM University Japan expert Lim Tai Wei noted that Ms Koike had, in 2008, failed in a bid to become LDP president – and thus, prime minister. He said: “If the (Olympics) organisation turns out successful, Koike can claim credit, perhaps enjoy public support and lay possible groundwork for higher political appointments in the future.”

Voter turnout was 59.73 per cent, up from the 46.14 per cent in the February 2014 poll.