The protest, scheduled for Friday, has been called by an alliance of seven parties in the capital, Kathmandu, in defiance of a ban on such rallies, to press the king to hand power back to an all-party government.
Prachanda, the Maoist leader, said in a statement: "Our party heartily appeals to the general public to defy the ban and demonstrate."
The elusive leader said his group was committed to supporting the political parties in their campaign against Gyanendra, who seized power last year saying that his intention was to tackle the decade-old Maoist revolt.
The Maoists, who want to install a communist republic in the impoverished Himalayan kingdom, and the main political parties recently forged a loose alliance to restore democracy.
Nepal's royalist government banned rallies on Monday anywhere in the capital, saying anti-monarchy guerrillas could infiltrate and cause trouble.
But Prachanda said the guerrillas had no plans to stir unrest. "We want to make it clear that we will not use any force in the rally, nor infiltrate it and create trouble," he said.
The opposition wants the king
to return power to the people
Security has been increased around Kathmandu for the rally, which is expected to draw thousands of people.
The Interior Ministry said in a statement: "All people are requested to carry any document that shows their identity while travelling in the public."
The Maoists killed 12 policemen in attacks around Kathmandu at the weekend in their biggest raid since they ended a four-month truce on 2 January.
More than 12,500 people have died since the revolt started in 1996.