Draped in dazzling kimonos, thousands of expensively made-up young Japanese women marked their entry into adulthood on Monday -- with many planning a night on the booze to celebrate.

Formal "Coming of Age" ceremonies, which began as a rite of ancient samurai families, were held nationwide for Japan's 20-year-olds, reminding them of their responsibilities after becoming old enough to legally drink and smoke.

As they fidgeted with mobile phones and stifled yawns during the speeches, the contrast in financial outlay between the sexes was obvious, with most males opting for the kind of plain business suit they will wear as future "salarymen".

"I'm happy I can finally drink alcohol and go clubbing," college student Rumiko Matsumoto told AFP while getting a $100 manicure in Tokyo's trendy Shibuya district ahead of the ceremony.

"When my nails are dry, I have to get my eyelash extensions done, do my hair and get fitted for my kimono.

"I'm very nervous," she added. "It's a special day, the first step towards being an adult.

"My parents told me I have to take responsibility for my own actions now. But first I want to celebrate by going drinking."

As more than four thousand gathered at Tokyo's Toshima-en amusement park on Monday, the fog of hairspray used to fix exquisitely coiffured perms hung in the cold air as young women queued for a rollercoaster ride.

Some women make appointments a year in advance to have their hair and make-up done and beauty salons often stay open all night to meet the rush for styling.

Many pay over $10,000 for their glittery kimonos, with beauty treatments such as elaborate nail decorations often costing hundreds of dollars more.

- 'Yikes, I'm an adult' -

"I did think 'yikes, I'm an adult' when I turned 20," said sales assistant Reiko Nakamura as a beautician fussed over her synthetic lashes.

"I have to think about my future so it's a little scary," she added, admiring her dagger-sharp Hello Kitty nails. "For now I just want to enjoy a night out drinking with friends I haven't seen since primary school."

Celebrated on the second Monday of the year from snow-swept northern Japan to the subtropical south, Coming of Age Day includes those who turned 20 over the previous year or will do so before March 31 this year.

The age a young person entered adulthood was set at 20 for both genders in 1876.

Crowds of kimono-clad ladies and suited young men offered prayers at Tokyo's Meiji Shrine over the holiday weekend, while thousands more flocked to Tokyo Disneyland and posed for photos with Mickey and Minnie Mouse.

In Japan's disaster-hit northwest, however, ceremonies were tinged with sadness as young people remembered classmates who perished in the 2011 tsunami.

There were an estimated 1.21 million new adults as of January 1 -- a decrease of some 50,000 from last year and half the 1970 peak of 2.46 million, according to government figures, mirroring the country's shrinking population.

Japan's annual birth rate recently dipped below one million for the first time in over 100 years, reflecting its rapidly ageing society.

But a population crisis was the last thing on Riki Hayashi's mind after a ceremony in Tokyo.

"The speeches went on for ages," he said. "We will all be getting drunk tonight."