Full movie is on Youtube:
Henry Fonda is the mastermind behind the plan, Larry Hagman plays the movie special effects guy. At ca. 0:18:41 starts the scene where Fonda meets Hagman on a movie set and they discuss the special effects.
At 0:45:25 the convoy transporting the gold bullions moves out.
At ca. 0:47:25 the robbers start the assault from the sewers.
At 0:52:40 explosions under the armored cars, simulated gunfire by special effects serves as distraction.
"The Alpha Caper" with Henry Fonda
There is a gold shipment in armoured trucks, and the robbers assault from the sewers.
The convoy of armored cars progresses slowly on the street, armed guards escort the convoy on foot, they don't expect a robbery attempt from below the surface, and I think the robbers even force the convoy to stop (maybe by manipulating the traffic lights?), they then remove the sewer covers and attach limpet mines to the hull of the cars from the sewers. They even build a mock-up of a sewer manhole and they practice how to attach the mine to the cars. They have a special tool for this.
That's all I remember, I saw this only once in the eighties.