Some sort of "disturbance" in the power grid forced the Turkey Point nuclear plant in south Florida to go into an emergency shutdown, said a spokeswoman for FPL Group Inc, the main energy provider in the area.

Other power companies were also affected as the outage spread sporadically but extensively up the Florida peninsula as far north as Tampa on the Gulf of Mexico. Officials called it a domino chain of blackouts.

A spokesman for Progress Energy's Florida utility said its clients were affected in the central part of the state.

Flights at Miami international airport were only marginally affected.

Linda Campbell of the Florida Reliability Coordinating Council said preliminary reports showed that the problem began at a substation in Miami-Dade County, leading to the loss of a transmission line and the shutdown of several plants owned by FPL subsidiary Florida Power and Light.

Mike Stone, a spokesman for the Florida Division of Emergency Management, said the blackout affected two to three million people while some local media said the number was higher.

Florida Power and Light said it expected to restore power to most Florida customers that lost power in about two hours.

"I don't know the cause of the outage," Stone told Reuters, saying authorities were waiting for an update from FPL. "There was a failure within the FPL system," he said.

Officials in Washington said there were no signs of a link to terrorism.

"There is no indication of a nexus to terrorism at this time ... we will continue to monitor," US Homeland Security Department spokeswoman Laura Keehner said.

FAILING TRAFFIC LIGHTS, STUCK ELEVATORS

Miami-Dade County Mayor Carlos Alvarez also assured residents there was nothing sinister about the incident.

"What I can assure people is that this was something technical. It wasn't anything criminal related," Alvarez told a news conference.

Lt. Elkin Sierra of the Miami-Dade fire rescue service said the biggest danger facing residents was failing traffic lights as people were not respecting the rules that say intersections should be treated as four-way stops when the signals are out.

"We have a high amount stuck in elevators. A lot of people are stuck in elevators," he added.

Miami-Dade police said no major problems had been reported. Hospitals switched automatically onto emergency generators.

"Remember this is south Florida. We are used to dealing with power outages because of hurricanes. This is like a dress rehearsal for us," said police spokeswoman Nelda Fonticiella.

CNN reported that eight power plants were shut down.

It was not just the Miami-Fort Lauderdale area, the most populous part of the state, that was affected.

The power also went out in Volusia County, around Daytona Beach, site of the famous racetrack, and in Brevard county, where NASA launches space shuttles from its Kennedy Space Center.

On Aug. 14, 2003, New York City and much of the Northeast and parts of the Midwest suffered a blackout that affected 50 million people. It was widely seen as the worst blackout in North American history.

That outage stranded hundreds of thousands of commuters and trapped subway riders underground in New York City, where thousands of people spent a hot night sleeping on sidewalks or walking miles in the darkness to reach their homes.

(Reporting by Scott DiSavino in New York, Randall Mikkelson in Washington, Jim Loney in Miami, Eileen O'Grady in Houston; Writing by Michael Christie, Editing by Frances Kerry)