Madrid (AFP) - Spain's "trial of the year" kicked off Tuesday with former lawmakers of the ruling Popular Party in the dock over one of the country's biggest corruption scandals.

Kickbacks, fake invoices and Swiss bank accounts: the juicy details of the so-called Gurtel affair -- which allegedly saw PP politicians across the country receive bribes -- are well-known to Spaniards who have become all too used to corruption in politics.

Spanish media have billed it as the "trial of the year", with 37 defendants facing justice, including two former party treasurers, notably Luis Barcenas, accused of stashing tens of millions away in Switzerland.

Another ex-treasurer had also been due to stand trial, but he fell seriously ill and will not appear.

As the defendants arrived at the court building, protesters shouted "Thieves! Crooks!".

The conservative PP itself has been called to the stand for allegedly benefiting from funds obtained illegally by lawmakers.

But unlike the defendants, the party as a whole faces no criminal charges and may merely be asked to repay the money.

- 'Godfather' figure -

The case has been called the "Gurtel" trial, the German translation of "belt", or "correa" in Spanish -- the surname of the man who allegedly led the sprawling corruption network.

Francisco Correa, a businessman who liked to be called "Don Vito" in reference to the mob boss played by Marlon Brando in "The Godfather", is accused of having showered PP lawmakers and councillors with bribes between 1999 and 2005.

In exchange, his companies or those of his friends were allegedly given contracts for public works or for organising events, such as the 2006 visit of Pope Benedict XVI in the eastern city of Valencia.

One of the defendants arriving in court Tuesday, Jose Luis Penas, a former PP councillor, secretly taped conversations with Correa and other defendants over two years.

His evidence is a major part of the prosecution case and will be seen as a mitigating factor should he himself be found guilty.

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Last year, online daily eldiario.es published a confession from Correa, who for more than three years swapped his flashy lifestyle for prison where he was remanded in custody from 2009 to 2012.

He said companies would give him "a commission of two to three percent" on the value of public works contracts awarded to them, which he would then give to Barcenas "in cash" after taking his share.

Correa, who was close to former conservative prime minister Jose Maria Aznar, has threatened to spill the beans on "everything he knew."

The case claimed a high-profile victim early on, forcing the resignation of then health minister Ana Mato in 2014.

Her ex-husband Jesus Sepulveda is on the stand for embezzlement of public funds and influence-peddling as former mayor of Pozuelo de Alarcon, a town near Madrid, from 2003 to 2009.

His family is alleged to have profited with various gifts, holidays or birthday parties complete with clowns.

Mato herself is accused of benefiting from her ex-husband's offences, though like the PP she does not face criminal charges.

- Rato in court next door -

The case has also claimed another victim in the form of Spain's superstar judge Baltasar Garzon, who ordered the first detentions in 2009 after a lengthy probe.

In 2012, he was suspended from the judiciary for 11 years after being found guilty of ordering illegal recordings of corruption suspects talking to their lawyers in the Gurtel affair.

The first day of the trial at Spain's National Court just outside Madrid was dedicated to procedural issues.

It is due to last several months.

Coincidentally, former IMF chief Rodrigo Rato also testified on Tuesday in a separate trial in the same building.

Rato, a former economy minister and fallen star of the PP, is on the stand for allegedly overseeing a "corrupt system" that helped him and other executives use millions in funds of banks he headed up for personal use.

He maintained that the whole system was "completely legal" -- an incentive that was part of his contract at Caja Madrid and a mere advance on salary at Bankia.