MIAMI — A south Florida nuclear reactor automatically shut down today, causing sporadic power outages throughout the state that affected 3 million people. Authorities did not specify the cause of the shutdown 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 Florida Power & Light nuclear reactors at its Turkey Point plant 30 miles south of Miami 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.

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

“It’s a matter of just a cascading effect,” he said.

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

“There are no safety concerns. The reactors shut down as designed,” Mr. Clark said. He said both reactors continued to have offsite electric power. He said two coal-burning power plants at Turkey Point also shut down.

Outages appear 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., though power in some affected areas had been restored an hour later.

In Collier County in the southwestern portion of the state, a 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,” 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,” a Volusia sheriff’s spokesman, Gary Davidson, said. “I know we’re receiving reports of traffic lights out virtually throughout the county.”

A spokesman for Miami-Dade County Department of Emergency Management, Jaime Hernandez, 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, a 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.