One witness said lava had burst across a road like a curtain of fire. He said he could smell sulphur and burning trees.
Community centres have been opened to provide 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.
Officials had been warning residents all week they should be prepared to evacuate as an eruption would give little warning.
Image copyright Reuters
Eyewitness Maija Stenback: ‘You could feel the eruption’
Leilani Estates, about six blocks away from the eruption.
Evacuated a couple of hours ago and we are now with friends. It’s me, my daughter, her boyfriend, and their two children, who are six years old and 20 months old. We all got shoved in a car and off we went.
Within about half an hour of the eruption, it went on social media, so me and my daughter went down to look at it. You could hear and feel the eruption a good half a mile away, and the closer you got, the more you could feel it.
It was like when someone plays the bass really heavy, and you can feel the bass – you could really feel the power and the lava – the colour of the lava was unbelievable, and the sound was unbelievable. It sounds very explosive, like something really explosive is coming out of a little hole, it’s spitting out as hard as it can. It’s not so much what you hear, it’s what you feel.
Hawaii’s Governor David Ige says he has activated military reservists from the National Guard to help evacuate thousands of people.
Skip Twitter post by @GovHawaii
Pictures showed a line of lava fountains from Mount Kilauea bursting up under a road and adjoining gardens.
Close-up footage shot by a drone showed lava emerging from a fissure in a wooded residential neighbourhood and oozing down a road.
Earlier this year, a false alert warning of an incoming ballistic missile caused panic, leading the US state to reassess its alert system.