French Eulex police are deployed on the Kosovo-Serbia border
The European Union has launched its mission to strengthen the rule of law in Kosovo, after months of delay.
Nearly 2,000 officials are taking over police, justice and customs duties from United Nations staff.
Serbia rejected Kosovo's declaration of independence in February and still regards Kosovo as one of its provinces, but it has accepted the new EU mission.
The UN will leave Kosovo after more than a decade in charge, retaining only a small political role.
Eulex is the EU's biggest-ever mission and will operate across all of Kosovo. The 1,900 international officials will be supported by about 1,100 local staff.
The EU will oversee the running of the police, the courts and the customs service.
The aim is to help the authorities deal with corruption and organised crime.
Eulex deployed peacefully to the main Serbia-Kosovo border crossing on Tuesday and traffic to and from Serbia continued as normal, the BBC's Nick Thorpe reports from the scene.
The customs post at Jarinje was set ablaze in February by Kosovan Serbs angry at Kosovo's declaration of independence.
But now the blue EU flag flutters there, with Belgrade's blessing, our correspondent says.
Eulex was supposed to have started months ago but it was plagued by delays and opposition from Serbia.
Belgrade has now accepted the plan, however many Kosovo Serbs are still unhappy that the UN is leaving.
How the EU force is being received in Pristina
The EU police say they are neutral on the status of Kosovo and are keen to show that they can deploy whenever and wherever they wish, our correspondent says.
About 20 Eulex officials entered the court building in the mainly Serb north of Mitrovica - a flashpoint town in the past, where Serbs and Albanians are separated by the Ibar River, the AFP news agency reports.
The mission is a huge test for the EU, says political analyst Lulzim Peci.
"If you succeed here, then you can prove to be capable to be engaged in other crises. But if you fail in Kosovo... then you lose credibility for overseas operations," he says.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.