Nepal's governing alliance has met former Maoist rebels to try to persuade them to rejoin the interim coalition.
Both sides say more talks are planned. The pullout is the most serious setback to peace moves since the Maoists ended a 10-year insurgency last year.
They withdrew on Tuesday in protest at the government's refusal to abolish the monarchy ahead of November elections to a constituent assembly.
Nepal's prime minister say the assembly must decide the monarchy's fate.
Analysts say the Maoists fear they will do badly in the polls.
Ahead of the talks, Dev Gurung, one of four Maoist ministers who resigned on Tuesday, said the former rebels were continuing to hold discussions with other mainstream political parties despite leaving the government.
"We can hold protests and talks in parallel," Mr Gurung, the former local development minister, told the AFP news agency.
"The door to discussion is always open. We are hopeful that they will rethink our proposition and seriously consider our demands."
Prime Minister GP Koirala has not yet accepted the resignations, which some analysts believe may be a Maoist pressure tactic.
Peace and Reconstruction Minister Ram Chandra Poudel, meanwhile, told the Reuters news agency that the government would try to convince the Maoists to return to the interim government and take part in the 22 November constituent assembly elections.
But Maoist leaders remain defiant.
"We are starting our door-to-door campaign on Wednesday and now we aim to bring about a republic from street protests," Ananta, a Maoist deputy commander, told AFP.
King Gyanendra's future is the centre of debate
He said that the rebels would now launch a three-week campaign of protests aimed at derailing the constituent assembly elections.
"All our sister organisations will be mobilised from Wednesday to ensure the constituent assembly elections are unsuccessful," he said.
On Tuesday, deputy Maoist leader Baburam Bhattarai warned there may be violence if the former rebels were prevented from holding protests.
The Maoist move raises fresh questions about how stable the peace process is in Nepal.
Their leaders have ruled out any return to armed conflict.
More than 13,000 people were killed during the rebels' 10-year fight for a communist republic. The civil war ended with a historic peace accord last November.
The fighting brought further poverty and misery to one of the poorest countries in the world.
Human rights groups regularly accused both the Maoists and the military of gross human rights abuses.
King Gyanendra was forced to surrender his powers in April 2006 after the Maoists joined forces with a coalition of seven political parties in a sustained campaign of street protests against his direct rule.
Some observers say the Maoists are short of options and say they may be in the process of trying to form an understanding with other left-wing parties.