SAN FRANCISCO — Millions of people in California woke up in the dark Wednesday after Pacific Gas & Electric started shutting off power to prevent what the utility called an unprecedented wildfire danger.

PG&E said it cut power to more than 500,000 customers in Northern California and that it plans to gradually turn off electricity to nearly 800,000 customers to prevent its equipment from starting wildfires during hot, windy weather.

A second group of about 234,000 customers will lose power starting at noon, the utility said.

The utility plans to shut off power in parts of 34 northern, central and coastal California counties to reduce the chance of fierce winds knocking down or toppling trees into power lines during a siege of hot, dry, gusty weather.

Gusts of 56-72 kph were forecast to sweep a vast swath of the state, from the San Francisco Bay area to the agricultural Central Valley and especially in the Sierra Nevada foothills, where a November wildfire blamed on PG&E transmission lines killed 85 people and virtually incinerated the town of Paradise.

The California Department of Forestry and Fire Prevention said it increased staffing in preparation for extreme fire weather.

The winds will be the strongest and most widespread the region has seen in two years, and given the scope of the danger, there was no other choice but to stage the largest preventive blackout in state history, PG&E said.

“This is a last resort,” said Sumeet Singh, head of the utility’s Community Wildfire Safety Program.

However, people should be outraged by the move, Gov. Gavin Newsom said. “No one is satisfied with this, no one is happy with this,” he said.

The utility needs to upgrade and fix its equipment so massive outages aren’t the norm going forward, he said.

It could take as many as five days to restore power after the danger has passed because every inch of power line must be checked to make sure it isn’t damaged or in danger of sparking a blaze, PG&E said.

The news came as residents in the region’s wine country north of San Francisco marked the two-year anniversary of deadly wildfires that killed 44 and destroyed thousands of homes. San Francisco is the only county in the nine-county Bay Area where power will not be affected.

To the south, Southern California Edison said more than 106,000 of its customers in parts of eight counties could face power cuts as early as Thursday as Santa Ana winds loomed.

The cutbacks followed a plan instituted after deadly wildfires – some blamed on downed PG&E transmission lines – destroyed dozens of lives and thousands of homes in recent years and forced the utility into bankruptcy over an estimated $30 billion in potential damages from lawsuits.

The outages Wednesday weren’t limited to fire-prone areas because the utilities must turn off entire distribution and transmission lines to much wider areas to minimize the risk of wildfires.