Mr Yeddyurappa, 75, is leading a BJP protest in the state in the name of an "anti-mandate day".
"Despite being rejected by voters, the Congress and JDS (Janata Dal Secular) are forming government. According to me they won't even last three months," Mr Yeddyurappa told NDTV.
"People of Karnataka gave a positive mandate to us. BJP won 104 seats and people rejected the Congress and JDS," he said.
The former chief minister also confirmed that no BJP leader would be present at the oath ceremony of Mr Kumaraswamy, who is expected to be sworn in along with a deputy from the Congress and a few ministers.
The BJP emerged the single-largest party in the Karnataka election but stopped short of an outright majority. The Congress, which lost power, moved quickly to tie up with the JDS and join hands in an arrangement in which JDS' Kumaraswamy would be chief minister but the Congress would get more ministries.
Many political observers and rivals have suggested that it will be hard for Mr Yeddyurappa to return as chief minister again. For one, he is past the party's "retirement age".
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