Belgian police have detained Marcel Brühwiler, chief executive of UBS Belgium, after allegations that the bank helped wealthy Belgians dodge billions of euros in taxes.
Brühwiler was detained at his office, while his house was searched by the police. He was released from police custody after about 12 hours of questioning in which he denied allegations.
"UBS helped rich people from Belgium get their money into Swiss accounts," a spokeswoman for the Brussels prosecutor's office said.
"And they actively recruited Belgian clients to put their money into this illegal system," she said, adding that the investigation would continue.
The prosecutor's office said it collected evidence from former compliance officers at the Swiss bank. It alleges that the bank started illegal activities, including money laundering, serious tax fraud and running a criminal organisation, 10 years ago.
The tax evasion is "estimated at several billion euros," according to the office.
However, a spokesman from the bank said UBS is fully cooperating with Belgian authorities and complies with all laws and regulations. He added that the bank does not encourage activities intended to help clients evade tax obligations.
The Zurich-based bank is facing a number of investigations over helping wealthy people evade income taxes. A similar probe in France resulted in fines of €10m ($13.6bn, £8bn) to the bank in 2013.
In 2009, the bank reached a deal with US authorities under which it will disclose information about US tax evaders and a pay $780m in fines.
It is also facing investigation from German authorities.
Swiss banks, who were notorious for their banking secrecy, are facing pressure from a number of countries that suffer from tax evasion amounting to several billions of dollars.
UBS's peer Credit Suisse Group recently pleaded guilty to helping Americans evade taxes, and settled a probe by US authorities with more than $2bn in fines.