Expectations have been high for the title, where users steer characters such as Bowser, Yoshi and even Mario's first love interest Pauline as they race karts through world cities, with New York the launch location.

"The servers are experiencing heavy traffic. Your log-in request will be processed in the order it was received," read a message that appeared when the game was launched.

A Nintendo spokesman said the company was aware that, due to heavy traffic, some users were having trouble accessing the game but he said the situation was improving.

Nintendo has been slow to expand into mobile gaming, choosing to focus on its hybrid Switch console. Its online subscription gaming business has lagged rivals Sony Corp and Microsoft Corp.

Mario Kart Tour, which risks disappointing gamers with its initial lack of a multiplayer option, includes an in-game currency that can be used to try and win rare characters and items and a $4.99 monthly subscription to unlock in-app rewards.

That's the same price point as Apple Inc's "Apple Arcade" streaming service, which offers more than 100 titles.

The game's teething problems came after Nintendo launched a lower-priced, handheld-only version of its $300 Switch console on Friday, aiming to provide an alternative to its aging 3DS device and to encourage people to buy multiple Switch units.

In Japan, the Switch Lite is estimated to have sold around 180,000 units in its first three days, based on data by industry magazine Famitsu.

The sales figures are a "spectacular start," Jefferies analyst Atul Goyal wrote in a note, adding that Switch Lite's lower price point was likely to drive demand for the device into the holiday season at the end of the year.

Nintendo's shares slid on Wednesday, closing down 4.3% ahead of the Mario Kart game launch. Its shares have climbed almost 40% this year.

(Reporting by Sam Nussey; Editing by Jane Merriman and Edmund Blair)

By Sam Nussey