Japanese billionaire Hiroshi "Mickey" Mikitani said Sunday he will donate $8.7 million to the government of Ukraine, calling Russia's invasion "a challenge to democracy".

The founder of e-commerce giant Rakuten said in a letter addressed to Ukraine's President Volodymyr Zelensky that the donation of 1 billion yen ($8.7 million) will go toward "humanitarian activities to help people in Ukraine who are victims of the violence".

Mikitani said he visited Kyiv in 2019 and met with Zelensky. 

"My thoughts are with you and Ukraine people," Mikitani said in his letter.

"I believe that the trampling of a peaceful and democratic Ukraine by unjustified force is a challenge to democracy.

"I sincerely hope that Russia and Ukraine can resolve this issue peacefully and that Ukraine people can have peace again as soon as possible," he wrote.

Russia's invasion has prompted broad financial sanctions from major democracies while individuals and organisations across the world have solicited donations to help Ukraine.

The Japanese government has also announced sanctions on Moscow including freezing assets and banning key exports such as semiconductors to organisations related to the Russian military.