The vote in Assam, in India's isolated northeast, starts a month-long election process in five states in the country's south and east.
The Congress party, which rules three of the states and heads the government in New Delhi, and its federal allies are fighting the national coalition's own communist allies as well as regional opposition parties.
In Assam, soldiers guarded about 515 candidates, their political workers and voters from attacks by separatists in 65 of the 126 constituencies across the state where polling was being held on Monday.
Voting for the remaining seats will take place on April 10.
A police spokesman said: "Strict vigil is being kept across the state to ensure peaceful voting, including helicopter patrols over remote areas to detect any rebel movement."
Though the rebels said they have nothing to do with the polls, the authorities were not taking chances as voting in past years has been marred by violence in the state.
The Congress government in Assam, which has a population of 26 million, is fighting for re-election against a regional opposition party and the Hindu nationalist Bharatiya Janata Party (BJP), India's main opposition party.
Opposition parties are highlighting the illegal immigration of Muslim migrants from neighbouring Bangladesh.
Assam, where a separatist revolt by the United Liberation Front of Asom (ULFA) group rumbles on, is the most populated in the turbulent northeast where more than two dozen insurgent groups are fighting Indian forces.
The Congress state government is banking on economic development in rural areas and Muslim voters who make up about 30% of the electorate to see it through.
The other states voting this month are the southern states of Tamil Nadu and Kerala as well as the federal enclave of Pondicherry and communist-ruled West Bengal in the east.