Christophe Lamfalussy added that one protester may have been injured.
The Serbs are protesting against the return of ethnic Albanians to homes in predominantly Serb areas of the city, which the European Union's EULEX security mission started to push ahead with last week.
Tensions flared on April 27 when EU police, backed by the NATO-led peacekeeping force KFOR, fired teargas at Serbian demonstrators attempting to enter an ethnic-Albanian area of Kosovska-Mitrovica. Tear gas was also used on Thursday as NATO forces and EU police tried to control around 50 people demonstrating in the divided city.
Kosovo's minority Serb population, which is dominant in the north of the territory, began to protest last week in Brdjani, a district of Kosovska-Mitrovica where the EU mission is trying to rebuild houses belonging to Albanians that were razed during the 1999 war.
The Serbs demand a ban on the return of ethnic Albanians until the same opportunity is provided to Serbs in the southern, Albanian-majority part of Mitrovica, which is split into mainly Albanian and Serb sections by the Ibar River.
"Several years ago the Serbs and Albanians agreed to draw a 'yellow line' dividing the Albanian Sukhoi district and the Serbian Brdjani district," a police official in Kosovo told RIA Novosti earlier. "They agreed to do nothing without agreeing with each other, but the Albanians didn't follow the agreement and started rebuilding their homes, ignoring the agreement."
According to the official, the Serbs insist the agreement is honored and oppose the Albanians crossing the yellow line.
Kosovo declared its independence from Serbia in February 2008, but has only been recognized by 56 of the 192 UN member states.