Dozens of customers officers in Turkey have reportedly been detained in a bribery investigation focused on Europe’s busiest border crossing.

A total of 38 officers were were arrested in a probe by the Edirne Chief Public Prosecutor’s Office into graft at the Kapikule border gate on the Turkey-Bulgaria frontier, the state-run Anadolu Agency reported.

A police surveillance operation set up in January discovered staff taking money from people entering and exiting Turkey at the crossing, according to Anadolu.

The addresses of the 38 suspects were raided, during which police allegedly uncovered European, US and Turkish currency to the value of more than €35,000. Arrest warrants were also issued for 350 people suspected of paying bribes.

According to Frontex — the EU’s border security agency — Kapıkule is the busiest border crossing in Europe.

It is the main land route into Turkey for Europe’s large Turkish diaspora, and about 4 million people and 400,000 vehicles enter and exit through the crossing every year.

In December, a similar anti-bribery operation was carried out in southern Mersin province in which 91 individuals were arrested, including customs employees and brokers.

Money and gold worth more than €150,000 were seized during the raid. Various items of gold jewelry, luxury watches, unlicensed firearms and drugs were also seized.

Turkey is currently in political turmoil, after the arrest of an opposition presidential candidate last month triggered the largest anti-government protests in over a decade.