GUWAHATI, India, May 13 (Reuters) - Three people were killed in India's restive Assam state on Sunday when ethnic Assamese clashed with tea plantation workers, officials said.

Authorities imposed a curfew in Doomdooma town in eastern Assam to prevent more violence in the tea-and oil-producing state, where thousands have died in the past three decades in ethnic and tribal clashes and separatist violence.

The three were killed after hundreds of tea plantation workers -- non-Assamese armed with spears and bows and arrows -- tried to break up a road block held by ethnic Assamese.

The Assamese put up the road block to protest the killing of a civilian during an anti-insurgency operation by Indian troops last week. The blockade had disrupted the supply of goods to tea plantations in the area, causing a scarcity of food.

Six more people were injured in the clash, police said, adding they had to fire in the air to disperse the mobs.

Separately, police fired on another group of Assamese protesters, critically wounding four people.

Assam, which produces a little over half of India's tea, is home to hundreds of thousands of tea plantation workers brought in from other parts of India by British colonial planters before the country's independence in 1947.

The biggest separatist group in the northeastern state is the United Liberation Front of Asom, which is fighting for the independence of the state from India, and claims to represent the ethnic Assamese people.

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