ROME  With tensions high on the eve of what is expected to be another round of large demonstrations here, the Rome police found a rudimentary, defective bomb under a seat in the subway on Tuesday, officials said.

The authorities isolated a subway stop, but did not shut down the system, and the mayor said the bomb would not have been able to explode.

A conductor found a package containing an assembly of tubes, wiring and a small amount of explosive powder about 10 a.m. as the train was at the outskirts of the city, the Rome transport agency said in a statement. A remote-control detonator in the package was missing a necessary part, said a spokesman for the Carabinieri, Italy’s paramilitary police force.