Chile’s government imposed a curfew and declared a state of emergency in response to a sweeping blackout that cut electricity to most of the country including the capital, Santiago, on Tuesday.

The massive outage, which began in the afternoon, affected eight million households across the country, from the northern port city of Arica to Los Lagos in the country’s south, officials said. In Santiago, it knocked out traffic lights, stranded people in elevators and shut down the subway network.

Hours later, the government announced a curfew from 10 p.m. to 6 a.m. in the regions affected by the outage. Schools in those regions will be closed on Wednesday, with about 300,000 students affected, officials said.

“Today has been a difficult day for millions of countrymen,” Gabriel Boric, the president, said at a news conference on Tuesday night.

By late Tuesday, power had been restored to about four million households, Mr. Boric said. But he warned that the recovery was slow and unstable, and the situation remained precarious.

He blamed the country’s power companies for allowing the outage to occur and for not restoring power earlier, adding, “This is outrageous.” The outage was caused by failure of a transmission system, officials said.

Soldiers and national police officers were sent to affected regions, officials said, and in Santiago, helicopters circled above the city.

Emergency services, hospitals, prisons and airports across the country were operating on backup electricity systems and generators, the national disaster agency said.

John Bartlett contributed reporting.