"A political era in Maharashtra politics has come to an end after the passing away of the Octogenerian Shiv Sena leader," Kumar said in a condolence message.
Thackeray had a special place in the political annals in Maharashtra, he said.
The RJD supremo Lalu Prasad and his wife Rabri Devi also mourned the death of the Shiv Sena leader.
Deputy Chief Minister Sushil Kumar Modi, the state unit BJP president C P Thakur and other party leaders have also mourned Thackeray's demise.