Former Barcelona president Josep Maria Bartomeu was arrested on Monday as part of a police investigation into last year's 'Barcagate' scandal, a source with knowledge of the case told AFP.

Bartomeu, who resigned as president in October, was among four arrests made just six days ahead of the club's new presidential elections on Sunday.

Barcelona's current chief executive Oscar Grau, head of legal services Roma Gomez Ponti and Bartomeu's advisor Jaume Masferrer were also arrested by Catalan police, who searched the club's offices on Monday morning.

Grau and Gomez Ponti were released late on Monday but Bartomeu and Masferrer would spend the night at a cell at a Barcelona police station before appearing before a judge on Tuesday, Spanish media reported.

Catalan police, the Mossos d'Esquadra, said in a statement the arrests were made as part of "an investigation into alleged crimes related to property and the socio-economic order" that has "been going on for almost a year".

Barcelona released a statement confirming the operation is linked to last year's 'Barcagate' controversy, when the club was accused of covering up payments made to a company called I3 Ventures, hired to boost the image of then-president Bartomeu on social media.

Part of the social media campaign included criticising current and former players, like Lionel Messi, Pep Guardiola and Xavi Hernandez.

Messi described the controversy as "strange" in an interview with Catalan newspaper Mundo Deportivo.

"FC Barcelona have offered up their full collaboration to the legal and police authorities to help make clear facts which are subject to investigation," the club said in its statement.

"The information and documentation requested by the judicial police force relate strictly to the facts relative to this case."

Spanish radio station Cadena Ser claimed Barca paid I3 Ventures an inflated fee and put payments through in smaller, separate amounts to avoid the club's financial controls.

Emili Rousaud, who resigned as Barcelona vice-president in March last year, said in an interview with RAC1 at the time: "If the auditors tell us the cost of these services is 100,000 euros and we have paid one million, it means someone has had their hand in the till."

The club took legal action against him.

Rousaud was among six Barca executives to leave their posts, with a joint letter citing the scandal as a key issue needing to be resolved.

'Plugging gaps'

Bartomeu maintained the company had been hired only to monitor posts on social media and announced an internal audit by PricewaterhouseCoopers, which cleared the club of financial corruption in July.

"Let one thing be clear," Bartomeu said.

"To the question: Have we commissioned the monitoring of social networks? The answer is yes.

"To the question: Have we commissioned to discredit people or institutions through social networks? The answer is no and we will take action against all those who accuse us of that."

Yet Bartomeu resigned in October, avoiding a vote of no confidence triggered after more than 20,000 club members signed a petition against him.

His departure came in the same month Barcelona announced losses of 97 million euros ($114 million) for last season and debts that had more than doubled to 488 million euros.

As well as a series of political blunders, Bartomeu had also overseen a dramatic decline in performances on the pitch and a personal falling-out with Messi, who tried to leave for free last summer.

Messi accused the club of "always juggling everything and plugging gaps" under Bartomeu's leadership.

Bartomeu's successor is due to be elected on Sunday, when club members will choose between the final three candidates, Joan Laporta, Toni Freixa and Victor Font.

"In light of events that took place today, we express our respect for the police and the judiciary, and we defend the presumption of innocence. And we deeply regret that these events diminish the reputation of the club," said Laporta.

"Too many people want to hurt Barca," wrote Freixa on Twitter.

"We will not allow it."