Opposition parties saw mixed results in Sunday's Upper House election, with the Constitutional Democratic Party of Japan (CDP) making big gains while others struggled to attract support in a crowded field.

The CDP, which ran on promises to postpone a planned consumption tax hike and raise the minimum wage to ¥1,300, won 17 of the 124 contested seats, gaining eight seats compared with its pre-election level to solidify its status as the leading opposition force in the Diet.

"In less than two years since setting out, we've been able to gain so much support," said party leader Yukio Edano.