BARCELONA (Reuters) - Hundreds of thousands of Catalans are expected to back independence from Spain on Sunday in a symbolic referendum on secession being held across the northeastern region, despite opposition from Madrid.

The "consultation of citizens" follows a legal block by the central government against a more formal, albeit still non-binding ballot which regional leaders had been pushing for.

Pro-independence organizations have campaigned vigorously for a big turnout from the wealthy region's 7.5 million people, and more than 40,000 volunteers will help set up informal voting stations on Sunday.

Pro-secession politicians hope a high level of support will prompt central government to sit down with them and negotiate more tax and political autonomy, or even convince Madrid to accept a full-blown independence referendum in the future.

Officials from Catalonia's two main parties, including the centre-right Convergencia i Union (CiU) of regional leader Artur Mas, have suggested that backing from more than 1.5 million citizens would help build momentum for their cause.

"The ideal scenario is the more people the better," Oriol Junqueras, head of left-wing opposition party Esquerra Republicana de Catalunya (ERC), said in an interview.

"It's clear that this consultation ... does not give us the democratic mandate we would have in an election, but what's important is that it is a fresh demonstration of the fact people want to vote, that they are keen to voice their opinion."

Opinion polls show that as many as 80 percent of Catalans want more autonomy from Spain, with about 50 percent in favor of full independence.

A long-standing breakaway movement in Catalonia, which accounts for one-fifth of Spain's economic output and has its own distinct culture and language, grew in strength during the recent years of deep recession.

In early September -- buoyed by a Scottish independence campaign which ultimately lost out in a referendum -- hundreds of thousands of Catalans dressed in the yellow and red of their regional flag packed the streets of Barcelona, forming a huge "V" to demand the right to vote.

Officially suspended by Spain's Constitutional Court after the Spanish government sought to stop this poll, Sunday's vote is nonetheless expected to pass off peacefully.

Government sources said it was unlikely the regional police, controlled by Catalan authorities, will stand in the way of people casting their ballots.

Analysts say the poll results should be viewed cautiously, with mostly pro-independence supporters expected to take part, while opponents are likely to shun it.

"While we expect the vote to have a symbolic impact (more than one million people will likely participate) it will not carry significant political implications," Antonio Roldan, Europe analyst at the Eurasia Group consultancy said in a note.