More than 200 Serbian protesters turned out to block traffic at the reopening of a bridge between the divided ethnic communities in Mitrovica, Kosovo.
The town was the flashpoint for serious violence between Serbs and Kosovo Albanians in March last year.
For the first time in more than a year private vehicles were allowed to cross between the mainly Serbian half to the north and the southern Albanian side.
Despite the protest, the UN will open the bridge for two hours each day.
Crossing times will eventually be extended. It is a small but symbolic step.
Mitrovica has to a large extent come to embody the ethnic divisions in the province.
On one side of the River Ibar lies the largest single enclave of Serbs in Kosovo, separated from Kosovo Albanians on the other by armed Nato peacekeepers.
The tentative reopening of the bridge came hours before the arrival of Kai Eide, a UN envoy sent to Kosovo to assess its readiness to begin negotiations on its final status.
The majority of the population wants independence but Serbs in the province want it to remain part of Serbia and Montenegro.