By Fiona Ortiz and Braden Phillips

BARCELONA, Spain (Reuters) - Spain's Catalans, angry over rising unemployment and persistent recession, were expected to deliver their separatist leader a mandate in Sunday's regional vote to press for secession.

Opinion polls show two-thirds of voters will vote for parties that want Catalan independence, and the election may therefore provoke a constitutional crisis over the legality of a referendum on independence.

Pro-independence flags, a star against red and yellow stripes, hung over balconies all over Barcelona, its capital city. Shop owner Margarita Bascompte told Reuters "in two weeks we sold more than we have in the last eight years".

Many Catalans believe they are taxed unfairly, crimping local spending on infrastructure and job creation. An estimated 16 billion euros ($21 billion) in taxes paid in Catalonia, about 8 percent of its economic output, is not returned to the region.

"Those who support (President Artur) Mas feel mistreated by Spain for a long time and we are fed up. The economic crisis has made the difference," said Rosabel Casajoana, 64, a teacher, emerging from a polling station having voted for CiU.

Mas's conservative Convergence and Union party, or CiU, is seen winning the most seats in the 135-seat regional assembly, or Parlament.

But the projected 62 to 64 CiU deputies is short of an absolute majority, so Mas - newly converted to separatism - will have to team up with smaller pro-independence groups such as the Republican Left, or ERC, to push ahead with a plebiscite.

That will put him on a collision course with Madrid, where Prime Minister Mariano Rajoy will use the constitution to block a referendum.

Home to car factories and banks that generate one fifth of Spain's economic wealth, and birthplace of surrealist painter Salvador Dali and architect Antoni Gaudi, the region also has one of the world's most successful football clubs, FC Barcelona.

With more people than Denmark and an economy almost as big as Portugal's, Catalonia has its own language. Like Basques, Catalans see themselves as distinct from the rest of Spain.

A recent convert to the cause of independence after a massive street demonstration in September, Mas campaigned on a promise to hold a referendum on secession.

CATALONIA IS BROKE

Catalonia's treasury is broke and the region's debt has been downgraded to junk. Blocked from the bond markets, Mas has had to seek billions of euros in rescue funds from the central government, itself fighting to prevent financial meltdown.

But, on the campaign trail, Mas focused on the region's gripes with Madrid. He told supporters he wanted to be the last president of Catalonia within Spain.

Voters said they felt this was the most important election since Spain returned to democracy in the 1970s after the Francisco Franco dictatorship.

The Catalan independence movement, which made a surprising comeback this year after decades of dormancy, has threatened Rajoy's mission to bring down painfully high borrowing costs by persuading investors of Spain's fiscal and political stability.

The recession and a high public deficit have pushed Spain to the heart of the euro zone debt crisis, and Rajoy is weighing asking for an international bail-out.

Wary that separatism could spread to the Basque Country and beyond, Rajoy said this week that the Catalan election is more important than general elections.

Alicia Sanchez-Camacho, the candidate for Rajoy's People's Party (PP) in Catalonia, warns of economic disaster if Catalonia tries to leave Spain. The PP looked set to be the second biggest party in Parlament with polls forecasting it will win 17 seats.

"Don't stay at home (on election day) if you don't want them to kick us out of Spain and out of Europe," she said at a campaign rally this week.

Some 5.2 million Catalans are eligible to vote in the polls, which opened at 0800 GMT and close at 1900 GMT.

MAS RISK

Enthusiasm for independence could ebb if voters think the price is having to leave the European Union, leaving Mas high and dry.

"I have no interest in independence. It's totally irresponsible," said 45-year-old Luis, a Peruvian immigrant and salesman who voted for the PP.

"It means exiting the EU and a drop in Gross National Product... Mas is an economist. He knows this but he isn't saying it. Why?" said Luis, who declined to give his last name.

After the vote Mas will struggle to push conflicting agendas: his promised referendum on independence and his drive to cut Catalonia's high deficit.

While the Republican Left may ally with him to push a referendum, it may pressure him to give up some spending cuts in exchange. The PP may support budget cuts but will try to block the referendum.

($1 = 0.7717 euros)

(Additional reporting by Elena Gyldenkerne; Editing by Louise Ireland)