MIAMI — A South Florida nuclear plant automatically shut down Tuesday, causing sporadic power outages throughout the state that affected up to 3 million people from Daytona Beach through the Florida Keys.

Authorities did not specify the cause of the midday shutdown of both reactors at Florida Power & Light's Turkey Point plant south of Miami but say there were no safety concerns.

Power was already restored in some places by early afternoon and was estimated to be fully restored by 6 p.m., Florida Power & Light said.

The Nuclear Regulatory Commission said the two reactors automatically shut down. Two other power plants farther north, the Crystal River reactor and St. Lucie twin reactors, continued to operate, although officials at those two facilities noticed the grid disturbance.

Miami-Dade County Mayor Carlos Alvarez said the outages were technical, not criminal.

"It's a matter of just a cascading effect," he said.

The outages have no connection to terrorism, Homeland Security Department spokeswoman Laura Keehner said.

"We don't know whether the grid disturbance caused the units to shut down or that their shut down caused the grid disturbance," said Kenneth Clark, a spokesman at the NRC regional office in Atlanta. He said the two reactors were automatically shut down.

"There are no safety concerns. The reactors shut down as designed," Clark said. Both reactors continued to have offsite electric power, and two coal-burning power plants at Turkey Point also shut down, he said.

Florida emergency management officials said the outages cut power to about 2-3 million people, although FPL said the number was closer to 800,000.

Outages appeared to be concentrated in the southeast portion of the state, including Miami, but were also reported in along the southwest coast and northeastern part of the state as well as in the Florida Keys. The outages began shortly after 1 p.m. EST, though power in some affected areas had been restored an hour later.

Several Miami-area hospitals switched to backup generators when the power went out. Miami-Dade schools were scheduled to be dismissed on time, and officials said school buses would be running.

In Miami's western suburb of Doral, Panera Bread bakery servers enjoyed the unexpected smoking break at the height of the midday rush-hour, while their manager grumbled over lost sales. At a Starbucks down the block, employees began handing out sandwiches they feared would go bad.

Nelson Suarez, 35, a manager for Asia sales at World Fuel Services, enjoyed the free lunch.

"I can't work anyway since all the power is out, so at least something good came out of this," he said.

In Collier County in the southwestern portion of the state, sheriff's spokeswoman Karie Partington said officials were working to determine the extent of the outages.

"We really don't have a good picture of it," sheriff's spokeswoman Karie Partington said. "It's not any one location."

In central Florida, the Orange and Volusia county sheriff's offices confirmed power outages at traffic signals across their jurisdictions.

"I don't have a handle on whether we're experiencing residential or commercial outages," said Gary Davidson, Volusia sheriff's spokesman. "I know we're receiving reports of traffic lights out virtually throughout the county."

Jaime Hernandez, a spokesman for Miami-Dade County Department of Emergency Management, said the county is partially activating its emergency operations center. He said no injuries have been reported so far.

By 2 p.m., most of downtown Miami appeared to be back to normal operation, including a campus of Miami Dade College and numerous stores and businesses. Traffic lights were out for a short time but appeared to be back in regular operation. In the Florida Keys, spokesman Andy Newman said areas were without power for about 15 minutes, but it was back up as well.

An official at the Miami International Airport says the facility is working on a generator backup but that no airline delays were reported.