Washington-area residents were clawing for the closest shade or air conditioner Tuesday as temperatures flirted with record highs in the first 24 hours of a two-day heat advisory.

The soaring temperatures are driving record increases in demand for power and emergency medical services, as well as interrupting construction projects and Metro service.

Temperatures peaked at 102 degrees at RonaldReagan Washington National Airport Tuesday -- one degree shy of its record of 103 set in 1999 -- and the National Weather Service predicts a high temperature of 100 on Wednesday, with the heat index making the day feel three degrees warmer.

D.C. resident Peter Armato, 56, said the heat has taken a toll on his Capitol Hill garden -- and him.

"Plants can sure tell the difference between 95 and 99," he said. Armato said he has taken plenty of precautions to avoid heat exhaustion while working outside -- but four hours in the garden Saturday forced him to take the next three days off.

"The heat is exhausting," he said.

In the past week, D.C. Fire and Emergency Medical Services has handled about 4,000 calls for service -- about a 25 percent increase from the daily average, which Fire Chief Kenneth Crosswhite attributed to the heat. On July 4, Fire and EMS responded to twice the average number of calls for service, Crosswhite said.

Construction worker Ivan Perez said his co-workers on the 2000 block of K Street Northwest were being sent home Tuesday to avoid heat stroke. And the stifling heat prompted the D.C. Road Runners to cancel a weekly run.

At home and in the office, residents are cranking up their air conditioning in tandem with the humidity -- sometimes too much.

Julie Rottier of Lovettsville, Va., said she thawed out at a park in Northwest Tuesday to escape her office's over-active air conditioner.

"I'm on my lunch break and I'm warming up," she said. "It's so cold in there, this feels good. I'm hot but I'm warming the old bones."

The region's three largest power suppliers -- Pepco, Baltimore Gas & Electric and Dominion -- are asking consumers to keep their blinds drawn and set their thermostats no cooler than 78 degrees to help relieve pressure on the region's power grid.

Pepco is expecting to break its record hourly usage peak -- which was set at 6,895 megawatts in August 2006 -- on Wednesday. Its customers in the District and Montgomery and Prince George's counties use roughly 5,300 megawatts per hour on an average summer day. One megawatt is enough to power roughly 1,000 homes.

The utilities have said they have enough power to manage the demand, but warned stress to the system increases potential for power failures.

If soaring demand critically threatens the system, Pepco could force a "brownout," in which the utility would cut off service for short periods, Anderson said.

Rails, like power lines, are expanding with the heat, causing trains to slow down as a precaution. Metro officials found a heat kink -- an area where an expanded rail pops out of place -- on the Red Line Tuesday evening. The MARC commuter line also has put its trains on heat restrictions.

Joey Flechas and Andrew Harnik contributed to this report.