Ukrainian President Volodymyr Zelenskyj said in a statement on Facebook that "gunfire has been heard and the bus is damaged"
What the man is looking for is still unclear, Ukrainian police say, writes Reuters.
Video sequences from the scene are heard automatically. The perpetrator is said to be equipped with both explosives and firearms, writes Reuters.
Police have cordoned off the central parts of Lutsk and TV images show empty streets closed off by people in military uniforms and police cars.
- Every effort is made to resolve the situation outside losses. I keep the course of events under personal control, says Zelenskyj.
Pictures on social media show a small bus parked on an empty street. Two of the bus' windows are broken and the other windows are covered with curtains.
The article is updated