TOKYO, July 21 (Reuters) - The fringe far-right Sanseito party emerged as one of the biggest winners in Japan's upper house election on Sunday, gaining support with warnings of a "silent invasion" of immigrants, and pledges for tax cuts and welfare spending.

Birthed on YouTube during the COVID-19 pandemic spreading conspiracy theories about vaccinations and a cabal of global elites, the party broke into mainstream politics with its "Japanese First" campaign.

The party won 14 seats adding to the single lawmaker it secured in the 248-seat chamber three years ago. It has only three seats in the more powerful lower house.

"The phrase Japanese First was meant to express rebuilding Japanese people's livelihoods by resisting globalism. I am not saying that we should completely ban foreigners or that every foreigner should get out of Japan," Sohei Kamiya, the party's 47-year-old leader, said in an interview with local broadcaster Nippon Television after the election.