LOS ANGELES, Sept 4 (Reuters) - A heat wave swept southern California for a sixth day on Sunday, claiming the lives of an elderly couple, setting record temperatures and leaving thousands of customers without power.

The mercury eclipsed the century mark throughout the area, sending millions of people to the beaches for relief on the Labor Day holiday. Weather forecasters expect more harsh conditions on Tuesday, but a slight cooling though the week.

In downtown Los Angeles, the temperature reached 99 degrees Fahrenheit (37 C), and set records along the coast in Long Beach (103 degrees/39 C) and 56 miles (90 km) inland in Riverside (112 degrees/44 C).

In the suburb of Valley Village, 13 miles (21 km) northwest of downtown Los Angeles, an unidentified couple in their 80s were found dead in their apartment on Sunday afternoon. Fire officials said the temperature inside was more than 110 degrees (43 Celsius). They said the man was disabled, and his wife may have been unable to reach the air conditioning control.

During the heat wave, 396,893 of Southern California Edison's 4.7 million customers -- 8.4 percent -- lost power "from a few minutes to longer," the Edison International <EIX.N>-owned utility said.

By late Monday, 9,000 customers of the city-owned Los Angeles Department of Water and Power's 1.4 million customers were reportedly without power.

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