The group, from all across Italy, had coverted a tractor into an amoured vehicle with cannon capabilities.
"The tank would have been transported out of the hangar and then a barge would take it to St. Mark's Square," Italian police colonel Michele Lo Russo said.
They're part of a growing separatist movement in the country's north, following an official referendum last month that saw 89% of respondents opt for independence.
Experts in Italian history believe the current Euro crisis has created a resurgence in North Italian nationalism.
"This type of separatist feelings now and then re-emerge over time, due sometimes to political upheaval or if you like, economic crisis," Dr Andrea Benvenuti said.
Many are frustrated that the prosperous north props up poorer regions in the south and that frustration has been compounded by the Global Financial Crisis and its impact on Italy.
A similar plot was quashed 17 years ago when demonstrators seized the tower bell in Piazzo Del Marco in Venice.
