The former rebels also said they would boycott the election process unless the monarchy was abolished.
While they warned of widespread protests, however, they also gave the interim government some breathing space – saying a nationwide general strike would not be launched until Oct. 4-6.
The Maoists spent years fighting for control of the impoverished kingdom and joined the political mainstream after King Gyanendra was forced to give up nearly all his power last year.
“Our protests that begin today will be peaceful, but we want to warn the government: If there are attempts to crush our peaceful movement, we will also get violent,'' the Maoists' deputy leader, Baburam Bhattarai, told 7,000 supporters in central Katmandu. He said the rebels would go door-to-door to tell people about their grievances, leading up to next month's general strike.
Bhattarai said the group was forced to leave the interim government because no progress was being made on its key demands, such as abolishing the monarchy and declaring a republic.
Later, another Maoist leader said they would boycott elections for a Constituent Assembly – which is to be elected in November to decide on Nepal's political system while the constitution is rewritten – unless the country immediately becomes a republic and abolishes the monarchy.
Chandra Prakash Gajurel said they would hold peaceful rallies and stop candidates from filing their nominations.
The former rebels accuse the king's supporters of trying to disrupt the elections and have been saying for months that balloting without ending the monarchy could not be free and fair. Gyanendra is still revered as a god in parts of Nepal, particularly in rural areas, although he wields no real political power.
There has been conflict within the Maoist movement over the role the former militants should play, said Lokraj Baral, a Katmandu-based independent political analyst.
“The Maoist leaders are under pressure from their activists because they have not been able to come up with an agenda that is distinct from the other parties, and show they are not just following around the other partners in the coalition,'' Baral said.
Leaders from the other parties said they would work to bring the Maoists back into the government.
“They have only separated themselves from the government, but are still part of the peace process,'' said Arjun Narsingh of Koirala's Nepali Congress Party. “We will continue to hold talks with them and try to address their demands.''
In addition to abolishing the monarchy, the rebels also demand a commission investigate disappearances of their supporters in their decade-long conflict, as well as the payment of salaries to former Maoist fighters, and their integration into the national army.
The rebels linked up with the political parties last year, joining pro-democracy street protests that forced Gyanendra to give up direct rule.
The Maoists later declared a cease-fire, signed a peace agreement and confined their combatants and weapons to U.N.-monitored camps. They joined Nepal's parliament in January and the government in April.