BOSTON -- Commercial traffic is being diverted off the Tobin Bridge after a crack was discovered, officials said Friday.

The crack on a transverse cross-floor beam on the upper level of the bridge was found during routine inspections. Commercial traffic traveling on Route 1 South will be diverted off the roadway at Route 16, according to Massachusetts State police.

"Inspectors and engineers are on site and investigating the root cause of this abnormality. Temporary and permanent repairs will be made as quickly as possible," said MassPort's Richard Walsh in a statement.

Officials said that the diversion will continue throughout Friday night and may be in effect Saturday morning.

The 57-year-old bridge spans the Mystic River between Charlestown and Chelsea. The bridge is a three-span cantilevered truss bridge.