News on thesandiegochannel.com:

The noninjury fire started just after 3 a.m. on some wood scaffolding during retrofitting construction on the west side of the bridge.

Electrical paneling inside the bridge caught fire and spread, causing plumes of smoke to billow from manholes along the span.

“The fire is in the wood scaffolding that’s inside the bridge that was used orginally to construct the bridge. And it has nothing to do with the support of the bridge because it was just left there when the bridge was rebuilt,” San Diego Fire Department’s Maurice Luque said.

…

The 405-foot-long, cantilevered bridge, also known as the Cabrillo bridge, was built for the 1915 Panama-California Exposition, celebrating the building of the Panama Canal.

Some small background: The San Diego Historical Society has some nice pictures of the bridge: under construction, another view, before it had the metal fencing to keep suiciders out — for a time it was nicknamed “Suicide Bridge.” There’s even a ghost story about it.

Apparently the 163 Freeway North and South is closed at the moment while Firefighters take care of the fire.

UPDATE: The 163 is reopened, but the bridge is still closed.