Voters trudged through rain to heavily guarded polling booths in India's rebel-infested Assam, the first of five states holding elections that will be the biggest test for the federal coalition since coming to power.

Assam, in the isolated northeast, is kicking off a month-long election process in states in the south and east.

Voters in tea-and-oil-rich Assam braved bad weather to reach polling booths and by early Monday afternoon about 30 percent of electors has cast their ballots.

Golak Thakuria, who says he is over 90 years old, has been voting in every election since 1947.

"I have been voting since India's independence and I have never missed an election," Thakuria said at Bengnabari, 60km west of Guwahati, the state's main city.

Police reported no violence in the state where thousands have died in

three decades of separatist and ethnic violence.

The Congress party, which rules three of the five states facing polls and heads the New Delhi government, and its allies are fighting stiff battles against the national coalition's own communist allies as well as regional opposition parties.

In Assam, soldiers with automatic rifles guarded around 515 candidates, their political workers and voters from attacks by separatist insurgents in 65 of the 126 constituencies across the state where polling was being held on Monday.

Helicopters flew over remote areas looking for rebels.

Voting for the remaining seats will take place on April 10.

Though the rebels said they have nothing to do with the polls, authorities were not taking chances as voting in past years had been marred by violence in the state of 26 million.