Authorities evacuated six buildings near the Manhattan side of the 59th Street Bridge yesterday after an underground electrical blaze threatened to ignite a gas line.

The fire broke out at about 2:50 p.m. while construction crews installed a water main under 59th Street between First and Second avenues, according to Deputy Fire Chief Thomas McKavanagh.

A short circuit ignited electrical insulation, and the fire began burning toward a low-pressure gas line running under the street, he said.

Most residents were away at work at the time, McKavanagh noted.

Utility workers shut off the gas line, which allowed the gas to safely bleed away.

Afterward, firefighters extinguished the smoldering insulation.