* Police say planned assault on Venice's St. Mark's Square
* Former parliamentarian among those arrested
* Pro-autonomy Northern League to protest against arrests
ROME, April 2 (Reuters) - Italian police arrested two dozen separatists including a former parliamentarian on Wednesday on suspicion of gathering arms to violently split the wealthy northern region of Veneto from the rest of Italy.
The group had converted a bulldozer into a tank with a makeshift cannon and intended to use it in an assault on St. Mark's Square in the region's capital Venice, police said.
Their plan recalled the 1997 seizure of the square's famed bell tower by armed separatists who proclaimed an independent Venetian republic, harking back to the city-state that was a major power in the eastern Mediterranean for centuries until it fell to Napoleon Bonaparte in 1797.
That assault was quashed in a matter of hours.
After searching premises in several regions of Italy, police arrested 24 people on suspicion of terrorism, subversion of the democratic order and making and possessing weapons of war. Twenty-seven others are under investigation, police said.
Those arrested included Franco Rocchetta, a former lawmaker and campaigner for Venetian independence who helped organise an self-styled "referendum" that caught attention abroad last month because it took place during the Russian annexation of Crimea.
The "referendum" was an online poll that said 89 percent of respondents voted for Veneto to break away from Italy.
Wednesday's arrests coincided with a vote in parliament to move to decriminalise clandestine immigration, enraging the Northern League which had backed the law that made it a criminal offence punishable by detention and fines in 2009.
The Northern League, which has long campaigned for the autonomy of Italy's rich northern regions, said it would rally in the northern city of Verona on Sunday to protest against the arrests and the softening of the immigration law.
Rocchetta was a founding member of the League, which has 4 percent of the seats in parliament, but has since left it.
"We call on the Venetians and all those who can no longer bear the intolerable oppression of the central state to fly the flag of St. Mark from their homes to show their solidarity with the Venetians and Lombards who have been unjustly imprisoned," Northern League Senate leader Massimo Bitonci said.
"A state that cancels the crime of clandestine immigration ... can only bring ridicule by imprisoning those ... who are fighting for the freedom of their people," he said. (Reporting by Naomi O'Leary; Editing by Tom Heneghan)