A veteran Tamil politician, M Karunanidhi, will be the new chief minister of the southern Indian state of Tamil Nadu.
Mr Karunanidhi will lead the state for the fifth time
Mr Karunanidhi's DMK party will head a new coalition government with support from the Congress party.
The DMK defeated the AIADMK party of former chief minister Jayalalitha.
Communist parties won two of four states where elections were held last month while the Congress party retained power in Assam and Pondicherry.
Newly-elected members of Mr Karunanidhi's DMK party elected him as their leader in the Tamil Nadu capital, Madras.
The Congress party, in power nationally, recorded one of its best election performances in Tamil Nadu in decades. It has decided to extend outside support to the government, which means they will not be part of the cabinet.
Mr Karunanidhi, 83, is now meeting the state governor to stake his claim to form the next government. It is the fifth time that he will take office as chief minister.
The outgoing AIADMK government was led by a former actress-turned politician, Jayalalitha - one of India's most controversial leaders - who has already resigned as chief minister.
But the biggest gainers of the elections were the country's Communist parties.
The Communists won a record seventh term in West Bengal
A Communist-led alliance won a record seventh term in West Bengal where it is already the world's longest running Communist government.
It also won power in the southern state of Kerala where it defeated the Congress party. Officials of the main Communist party, the CPI(M), will meet in Delhi on Saturday to name the new Kerala chief minister.
Separately, the Congress party retained power in Pondicherry and Assam - where it fell slightly short of an outright majority and is being supported by a party of Bodo tribes people, the Bodoland Peoples' Progressive Front.
Congress also celebrated after a massive win for party president Sonia Gandhi in the parliamentary constituency of Rae Bareilly which she won by a margin of 417,000 votes.
She quit the seat in March over a political row but has been returned by an even bigger margin than before.