The Maoists have ordered factories, schools, colleges and public transport to shut down on January 20 to protest at what they say are the ruling parties' plans to hold a parliamentary vote on disputed issues in the draft constitution.
The Maoists, now in opposition, say discussions must continue until a deal is hammered out -- even if that means missing theJanuary 22 deadline.
"We are trying to forge consensus... there has to be consensus," said Mr Pushpa Kamal Dahal, known better as Prachanda, at a press conference.
"If the coalition takes a forceful step towards a (vote)... the country will move towards political conflict, and we cannot let that happen," Prachanda said.
Nepal has endured prolonged political limbo since 2006, when the Maoists ended their decade-long insurgency and signed a peace deal which paved the way for elections two years later.
Despite extensive discussions since a November 2013 election and the appointment of a new prime minister last February, political parties have failed to make headway on disputed issues in the national charter.
A key sticking point concerns internal borders, with the opposition pushing for new provinces to be created along lines that could favour historically marginalised communities such as the "untouchable" Dalit caste and the Madhesi ethnic minority.
Other parties have attacked this model, calling it too divisive and a threat to national unity.
Police arrested more than 70 protesters last week for attacking vehicles or coercing shopkeepers to close their stores during a Maoist-led strike in the Himalayan nation's capital.
Prachanda warned security forces against cracking down on his supporters on January 20, saying the state would "be responsible for the consequences".
The strike is backed by a hardline group which split from the main Maoist party in June 2012, accusing its leaders of betraying their radical principles.
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