One witness said lava had burst across a road like a curtain of fire. He said he could smell sulphur and burning trees.
The American Red Cross has opened an evacuation shelter.
The eruption follows a series of strong earthquakes over recent days.
A volcanic crater vent - known as Puu Oo - collapsed earlier this week, sending lava down the mountain's slopes towards populated areas.
Hawaii's Governor says he has activated the Hawaii National Guard.
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