No injuries were reported.
Authorities ordered the evacuation in the San Miguel region after the eruption Saturday, ElSalvador.com reported Sunday.
The volcano erupted at 10:30 a.m. for the first time in 37 years.
Residents reported hearing what sounded like a bang followed by an explosion, then seeing a thick column of ash spew into the air.
The Directorate of Civil Protection declared an orange, or high-risk, alert for the region.
When officials reported Chaparrastique had been showing increased activity since Dec. 13, some residents of San Miguel and San Jorge Chinameca began to seek safety elsewhere, ElSalvador.com said. Others, however, said they were caught unaware.
"We caught by surprise," said Maria del Carmen Gomez, 74, who lives in Canton Las Placitas in Chinameca.
Authorities also were caught flat-footed, ElSalvador.com said, reporting that officials couldn't agree on how to address the emergency 3 hours after the eruption began.
"The authorities were slow. We got ready our things, but we never moved," said Rosa Recinos of Miramar. "For our children, it is a difficult situation."