A trash truck with a metal rod on top snagged the power line at about 7:15 a.m. and brought it down on Interstate 710 in Commerce, east of downtown Los Angeles.
"At that point, the wires were hot," California Highway Patrol Officer Monica Posada told City News Service.
The line later got caught on a big rig. It also caused a fire in some grass and brush just off the freeway but the blaze was quickly doused and no injuries were reported, Los Angeles County fire officials said.
About 1,600 Southern California Edison customers lost electrical power for more than 1 1/2 hours, spokeswoman Jennifer Manfre said.
The freeway is a main truck route to and from the ports of Long Beach and Los Angeles and also connects with Interstate 5, a main route between Los Angeles and Orange counties.
The accident happened away from off-ramps, and miles of cars and trucks were stuck. TV news reports showed drivers leaving their cars and walking on the road. A line of motorists stopped to snap cellphone photos of the downed line as a worker began cutting the line.
The roadway reopened after more than an hour.