* State anti-corruption unit leads investigation
* 'Crowdfunding' raises 15,000 euros in 24 hours
* Protestors hope to take Bankia to court
* Mainstream political parties shy away from legal action (Recasts with public prosecutor investigation)
By Sarah White
MADRID, June 6 (Reuters) - Spain's public prosecutor's office opened an investigation into nationalized bank Bankia on Wednesday, potentially heightening a backlash against the lender's bailout that has so far been spearheaded by small shareholders groups.
The public prosecutor's anti-corruption unit is leading the probe into Bankia, a spokesman said, the first step in determining whether an offence has been committed by the bank that has requested 19 billion euros ($24 billion) in state aid.
The investigation comes as public anger at how the listing of the bank last year and its nationalisation last month have been handled by Spanish authorities.
Campaigners seeking to launch legal action against bailed-out bank are attracting donations via crowdfunding, a financing method all the rage in small business circles that involves drawing cash from ordinary citizens.
Protestors from the "indignados" movement - a campaign of Spaniards "indignant" over the government's handling of the economic crisis - had by Wednesday raised 15,000 euros in 24 hours through a donations website to launch a claim.
The group said it plans to start civil and criminal proceedings against the former management of Bankia parent BFA and former chairman Rodrigo Rato who stepped down in May shortly before the bank requested 19 billion euros in state aid.
Bankia, weighed down by souring real estate assets, has become the focal point of a Spanish banking crisis that threatens to force the country to seek international help.
Thousands of small shareholders, drawn into Bankia's July listing after an aggressive marketing campaign on television and in branches, have seen their investments wiped out as the bank's stock tanked, adding to public anger.
The challenge mounts pressure on politicians to open a formal investigation into the handling of Bankia, with other minority shareholder groups also stepping up plans for a legal fight against former management and regulators.
Over 500 Bankia minority investors are set to club together to try to seek compensation, and will start planning their challenge next week, one association for small shareholders of public companies, AEMEC, said on Wednesday.
Some smaller political parties have also called for a criminal investigation into the Bankia affair.
However, the two main political parties have so far shied away from launching an inquiry run by Congress. Former Chairman Rato was a previous finance minister for the ruling People's Party, while the roundly-criticised listing of Bankia happened under the auspices of a Socialist government, now in opposition.
The Indignados group said it would present its legal challenge on June 14.
The protestors, best known for marches and campaigns organised across Spanish towns in the past year, used a website called Goteo, or "Trickle", to raise the funding, explaining they needed money to pay lawyers and administrative costs.
Crowdfunding has become an increasingly popular method of financing start-up businesses or even independent films. In many cases though, this so-called peer-to-peer lending implies investors will get some sort of a return.
Those leading the legal challenge against Rato said the campaign was for justice, not for money.
$1 = 0.8023 euros Additional reporting by Andres Gonzalez and Tracy Rucinski; Editing by Julien Toyer and Mark Potter