Thousands have fallen ill, transport has been disrupted and schools closed after days of fires in Riau province on Sumatra island, where blazes are deliberately lit every year to clear land for palm oil and wood pulp plantations.
More than two dozen people suspected of starting fires in rainforest and peatland have so far been arrested, said national disaster agency spokesman Sutopo Purwo Nugroho.
Haze from fires on Sumatra is an annual problem in Southeast Asia, but last June Singapore and Malaysia were cloaked in the worst smog for more than a decade.
While some haze was detected in the two neighbouring countries in recent days, the air quality was mostly good.
Declaring an emergency allows Riau to seek help in tackling the blazes from the central government, and Nugroho said aircraft were preparing to drop water on fires and carry out "cloud-seeding" to chemically induce rain.
"The disaster agency is preparing aeroplanes and helicopters to carry out water-bombing of the fires," he said.
More than 25,000 people have fallen ill in recent days due to high air pollution, with most suffering respiratory tract infections, said Riau disaster chief Said Saqlul Amri.
The emergency status means health centres must see patients free of charge, he said.
AFP