Dutch prosecutors say they have launched co-ordinated raids in several countries against suspected money-launderers and tax evaders.
They are investigating about 3,800 Dutch-linked accounts in an unnamed Swiss bank following a tip-off they could contain undeclared assets.
Paintings, a gold bar, cash, a luxury car and jewellery have been seized.
As well as the Netherlands, there have been searches in France, Germany, the UK and Australia.
The Dutch government has passed information to the other countries about more than 50,000 suspect accounts at the bank.
UK authorities said the investigation was also targeting senior bank employees.
Ill-gotten gains? Anna Holligan, BBC News, The Hague
The suspicion is that these hidden assets are largely ill-gotten gains, the proceeds of crime. The Dutch financial investigations service has highlighted the unfairness and corrupting nature of tax evasion.
While the practice is apparently becoming more prevalent, the authorities say so too is the exchange of data between governments, the use of smart data to track payments and the use of informants.
The Dutch authorities have warned that the international co-operation is making it harder for the secret bankers to remain below the radar.
But in taking this co-ordinated action without apparently consulting their Swiss colleagues, the Dutch have diluted what might otherwise been seen as a EU-co-ordinated triumph.
The co-ordinated raids on suspects' properties began on Thursday, the Dutch office for financial crimes prosecution (FIOD) said in a statement.
The cross-border operation is being supported by Eurojust, the EU's judicial co-operation unit, which said the suspect funds ran into the millions of euros.
Credit Suisse bank has released a statement saying its offices in London, Paris and Amsterdam "were contacted by local authorities concerning client tax matters. We are co-operating with the authorities".
It said it "continues to follow a strategy of full client tax compliance".
However, it did not explicitly confirm that it was the institution at the centre of the investigation.
The Swiss bank has already been in difficulty over its customers' taxes in the United States, says the BBC's economics correspondent Andrew Walker. In 2014 it agreed to pay a fine of $2.6bn (then £1.5bn; €1.9bn).
But the Swiss attorney general expressed annoyance that it had not been informed in advance about the operation.
"The Office of the Attorney General of Switzerland is disconcerted about the manner in which this has been organised with the conscious non-inclusion of Switzerland," it said in an emailed statement.
"The applicable practices and rules of international co-operation and legal assistance have obviously not been complied with here.
"The attorney general expects a written explanation from the relevant leading Dutch authorities and is examining further actions," the OAG said.
The Netherlands raids targeted homes in four locations - The Hague, Hoofddorp, Zwolle and the municipality of Venlo. Two people were detained for interrogation.
In Australia, authorities said investigators were looking for the owners of 340 anonymous bank accounts.
The UK tax inspection body, HMRC, confirmed that it had launched "a criminal investigation into suspected tax evasion and money-laundering by a global financial institution and certain of its employees".
"The first phase of the investigation, which will see further, targeted, activity over the coming weeks, is focused on senior employees from within the institution, along with a number of its customers," HMRC said.
"The international reach of this investigation sends a clear message that there is no hiding place for those seeking to evade tax."
The investigation will continue for weeks to come, Dutch financial prosecutors said.