Hero cops managed to block all traffic from Baltimore’s Francis Scott Key Bridge with just seconds to spare before a massive container ship slammed into it — then watched in horror as it crumbled into the frigid waters.

“Hold all traffic on the Key Bridge,” one emergency dispatcher told officers around 1:30 a.m. Tuesday, according to police radio traffic. “There’s a ship approaching that just lost their steering.”

Two cops who happened to be nearby responded, and stationed themselves at either end of the 1.6-mile span — stopping cars from crossing while the 130,000-ton Dali cargo vessel barreled out of control toward the bridge after apparently losing power.

The cops noted that a pothole repair crew was still on the bridge, apparently on a meal break. But before they could warn them, the Dali collided with a support beam in the middle of the Patapsco River — causing the entire bridge to collapse within seconds — and throwing eight workers into the water.

“The whole bridge just fell down. Start, start! Whoever, everybody! The whole bridge just collapsed,” one frantic cop radioed in.

“The bridge is down!” another officer shouted.

This is a developing story.