Monday's ambush represents a stepping up of violence that analysts says has dimmed hopes for early peace talks.
The attack, the biggest since Sher Bahadur Deuba was reappointed prime minister on 2 June, took place on a highway in a forest area at Khairikhola, 450km west of the capital Kathmandu.
Surviving policemen said hundreds of rebels surrounded the police convoy and fired automatic weapons at the first truck. Soon after, there was a blast under the second truck.
A police official said the trucks were carrying 37 policemen on their way to clear a roadblock put up by the Maoists in the area. Six of the wounded were in critical condition, doctors said.
Officials said they did not have any information about rebel losses as the Maoists usually carry away their fallen comrades.
The Maoists have been fighting since 1996 to overthrow Nepal's constitutional monarchy and set up a communist republic in the world's only Hindu kingdom.
"They (the Maoist rebels)see the new government as a new equation by the king to crush the revolt and would therefore carry out more attacks"
Editor, Himal magazine
More than 10,000 people have been killed in the insurgency that has wrecked the economy of the poor Himalayan country wedged between Asian giants China and India.
Soon after King Gyanendra reappointed him, Deuba appealed to the rebels to resume talks they abandoned last year.
But the Maoists, who consider Deuba to be a puppet of the king, rejected his call and said his reappointment would exacerbate the bloody conflict.
The rebels have since then set off two powerful bombs at a school in Kathmandu causing extensive damage but no casualties.
They have also forced 6.5 million students to stay home for more than a week by calling an indefinite school strike across the country.
The attack was the biggest since
Deuba was reappointed PM
The bomb blasts and school shutdown should have warned the government of attacks like the one on Monday, said Rajendra Dahal, editor of the widely read Nepali magazine, Himal.
"They see the new government as a new equation by the king to crush the revolt and would therefore carry out more attacks," Dahal said.
Meanwhile, Nepali officials said on Monday that India had provided two military attack helicopters to the country's poorly equipped army to help crush the insurgency.