The fire began in the afternoon and flames were spread quickly by high winds blowing toward the Pacific, leading authorities to declare a state of emergency for both cities.
Deputy Interior Secretary Mahmud Aleuy said that about 4,500 people in six neighborhoods had been evacuated as flames advanced nearby and that an additional 10,000 might need to be moved.
Aleuy said some buildings had been burned but the extent of damage had not yet been determined.
Chile suffered its worst urban fire in the same general area a year ago.
In April, a raging blaze that started as a forest fire leaped from hilltop to hilltop in Valparaiso, killing 15 people, injuring more than 500 and destroying more than 2,900 homes.
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