The worst blackout in more than a decade hit Athens and southern Greece on Monday, leaving millions sweltering in a heat wave and raising concerns about whether the lights will go out at next month's Olympics.

The government blamed the outage on "mismanagement" of the electricity grid, and officials promised the network was ready to handle the Olympics.

But it was another hurdle in Athens' attempt to convince the world it is ready to host Games that are well-run and safe. Preparations have come under criticism because of construction delays and concerns over security arrangements to stop terror attacks.

The blackout incapacitated air conditioners as afternoon temperatures soared to 104 degrees. It also created enormous traffic jams from failed traffic signals and stalled electric trolleys. Hundreds of passengers on the Athens subway were forced to leave trains and walk, and the fire department received hundreds of calls about people trapped in elevators.

In one embarrassing moment for the government, the transport minister, Michalis Liapis, was stranded when the power failed as he was making a test run to showcase a new Olympic rail link from central Athens to the airport.

The outages were traced to an imbalanced flow of electricity that shut down four power-generating stations, according to a senior government official who spoke on condition of anonymity.

Greece's Public Power Corp. did not explain what caused the blackout, saying only that it knocked out four major plants. The company, an Olympic sponsor, blamed the state-owned grid operator for the outage.

The development minister, Dimitris Sioufas, said there was more than enough power for a "smooth and uninterrupted" flow of electricity to the whole country during the Olympics. He also noted that five additional electrical substations were to go into operation next month to lower the chances of power spikes.

Outages were reported as far away as Larissa, 135 miles northwest of Athens, and the port of Kalamata, 110 miles southwest. The blackout also hit some islands in the Aegean and Ionian seas.

Power was restored to 70 percent of the region in about an hour and to all of Athens in just more than three hours. Remote areas were affected longer.

Passengers wait for a train at an Athens subway station during a blackout across southern Greece that created traffic jams and left millions sweltering in a heat wave.