A power outage shut down two subway lines in Brooklyn yesterday amid a heat wave in New York City.

A Con Edison official said the outage originated at a smoking manhole at the intersection of Smith and Dean streets in downtown Brooklyn.

The 2, 3, 4, F, and G trains were affected by the loss of power, which hit just after 5 p.m. The F and G were shut down completely in sections, while the other trains were slowed.

No passengers were reported stuck in the trains. The electrical outage affected signals, not the third rail, meaning trains were able to move and had air-conditioning and lights.

An official said the MTA was using portable generators as a temporary fix until the utility could restore power.

Temperatures soared to 94 degrees yesterday, prompting warnings from city officials to conserve energy.

A spokeswoman for Con Edison, Elizabeth Clark, said it was too early to speculate whether the hot weather was the cause, and she said crews had begun an investigation.

The utility has come under fire in recent years after a blackout during a heat wave in 2006 left a swath of Queens without power for more than a week.

Council Member Eric Gioia, who became a vocal critic of the utility following the 2006 blackout, said he was concerned about a repeat this summer.

“New Yorkers have gotten used to shoddy service and periodic outages from Con Edison,” he said.

“I would recommend that New Yorkers keep bottled water, batteries, and hard-line phone in their home, and prepare for the worst.”

In response, Ms. Clark said the utility had invested $1.7 billion in the electric system “to prepare for summer 2008.”

“We are constantly updating our system,” she said.