"Three of the wounded policemen have been rushed to a nearby hospital with head injuries," Shiva Nepal, a district official, said.
"The Maoists ransacked the post, threw out belongings and utensils of the policemen before leaving the area," he added.
Maoist leaders were not immediately available for comment.
Police posts opened
Several Maoists demonstrators were reported to have been injured in this incident and in other violence on Sunday.
Nearly 2,000 police posts, closed during the 10-year anti-monarchy Maoist revolt, are being reopened across Nepal after rebels and the government signed a peace deal in November.
On Sunday, authorities imposed a curfew to prevent further clashes in the southeastern town of Lahan, where a 16-year-old boy was killed when a former rebel shot at a crowd of protesters two days ago, another official said.