LOS ANGELES (CNN) - About 700,000 electric customers in Los Angeles lost power Monday afternoon after a worker mistakenly cut a wrong line, triggering a cascade of problems in the city's power grid, a spokesman for the Los Angeles Department of Water and Power said.

About 50 percent of the DWP's 1.4 million customers were affected by the outage, which began about 1 local time (4 pm ET). Power had returned to most areas an hour and a half later, according to DWP spokesman Ron Deaton.

The blackout trapped people in elevators and caused traffic to snarl at intersections where stoplights weren't working.

According to Deaton, after the line was mistakenly cut, other lines in the area could not handle the additional load, triggering a string of problems.

Two stations in the San Fernando Valley that connect the generation and distribution systems tripped off line, causing three power plants to shut down automatically as a safety measure, according to city officials. The outages stretched from the valley through downtown south to San Pedro, as well as parts of the city's west side.

After the power failed, the Los Angeles Police Department went on full tactical alert, with officers ordered to stay on duty at the end of their shifts. The alert was later downgraded, although officers were still being kept on duty to assist in traffic control.

The Los Angeles Fire Department received numerous calls from people trapped in elevators, said department spokeswoman Jim Wells.

Los Angeles International Airport reported flickering of lights but no interruption of flight operations. Van Nuys Airport, a busy general aviation airport in the San Fernando Valley, lost electricity but was able to continue operations with backup power.

-----------------------------

Read about talk of hydrogen cars in California.

--CNN Correspondents Jen Rogers, Ali Velshi and Jeanne Meserve contributed to this report