Police sources say the officers were beaten and forced to give up their weapons.
Tension was high in the town as several hundred Serbs continued a third day of protests to push for the release of a Serb, arrested for setting fire to a car owned by an ethnic Albanian.
Bernard Kouchner, the head of the UN's Kosovo mission (Unmik) flew in to Mitrovica on Wednesday to hold crisis talks with local officers.
He was accompanied by the commander of the Nato-led K-For peacekeeping force.
UN under attack
In the most serious attack on UN forces, a mob of dozens of Serbs stormed into and looted the apartment of two UN police officers on Tuesday.
Police sources say the officers were beaten and forced to give up their weapons.
In another incident, a UN vehicle was blocked and attacked with an iron bar.
Staff at the UN courthouse were evacuated after midnight on Tuesday, UN spokeswoman Susan Manuel said.
Mr Koucher made it clear he deplored the attacks on UN personnel.
"He is outraged that the Unmik police who live and work among the Serb community for their benefit and protection have been targeted and abused," Ms Manuel said.
Patrols suspended
All UN police patrols have been suspended since the attacks, UN spokesman Mike Keats said.
The evening curfew has also been set two hours earlier time at 2200 local time.
However, the international peacekeeping force is on high alert, with UN police being kept on standby in the police station or their accommodations.
The Nato-led force was boosted on Wednesday, with seven armoured personnel carriers and at least 20 soldiers dispatched to the main intersection of the town.
Peacekeepers have also been maintaining a strong presence on the bridge over the Ibar River, which divides the Serbs in the north from the ethnic Albanians in the south.
Mitrovica has often been a flashpoint for clashes between Serbs and ethnic Albanians.