The rebels got close to the police post on Saturday evening while pretending to be a wedding party, then attacked in Mahendranagar town, about 300km southeast of the capital, Katmandu.
A 30-minute gunbattle ensued, said police official Topendra Hamal. When soldiers reached the area, the attackers fled, carrying away their wounded.
One policeman was killed, and the five in critical condition were flown to Katmandu for medical treatment. It was not clear how many rebels were killed, Hamal said.
The rebels, who say they are inspired by Chinese revolutionary leader Mao Zedong, have been fighting since 1996 to replace Nepal's monarchy with a communist state. More than 10,500 people have died since the conflict began.
In a seperate development, thousands of Nepalese on Saturday protested against King Gyanendra in the Indian capital, demanding the restoration of multi-party democracy in the Himalayan kingdom.
The protesters carried placards and red flags and chanted slogans for the restoration of democratic rights in Nepal.
Protesters chanted slogans for
restoration of democratic rights
"It's a do or die situation," said Sujata Koirala, daughter of Nepali Congress party president GP Koirala. "Either we will win or he (the king) will win."
She said Congress "saved" the institution of monarchy for 60 years, "but now we are changing our minds. People have lost their confidence in the king. Nobody wants monarchy anymore."
Koirala, who said she escaped to India on a motorcycle, and other Nepalese leaders demanded the forming of a constituent assembly and fresh elections.
They said they were looking to India for help. "There is no hope in Nepal. We need the help of India, the United States and other countries," Koirala said, while urging New Delhi to stop all military aid to Nepal.