In a statement tweeted on Monday, Los Mossos d'Esquadra confirmed "several searches are being carried out" at Barca's club premises by its unit dedicated to the investigation of economic crimes.
According to the EFE news agency, Bartomeu has been taken into custody as part of the probe, along with the club's current CEO Oscar Grau.
Barca's head of legal services Roman Gomez Ponti and former director Jaume Masferrer were also arrested, EFE said.
The investigation is thought to centre around the "Barcagate" scandal, when Barcelona allegedly paid social media company 13 Ventures to smear celebrated club names such as Lionel Messi, Pep Guardiola, Xavi, Gerard Pique and ex-president Joan Laporta during Bartomeu's leadership.
Los Mossos searched Barca's offices in July 2020 in relation to the claims, although the club hired Price Waterhouse Coopers to investigate the matter and the global auditing firm found in their favour.
Nevertheless, board members Emili Rousaud, Enrique Tombas, Silvio Elias, Josep Pont, Maria Teixidor, Noelia Ronero and Jordi Calsamiglia resigned amid the damaging fallout.
Laporta is favorite to return for a second stint as president, with elections taking place this Sunday – the race now staged against the backdrop of yet another turbulent episode for the LaLiga giants.