Nepal sank into political turmoil after politicians failed to agree on a new constitution, leaving the country with no legal government.
Prime minister Baburam Bhattarai called new elections, but critics said he lacked the power to do so.
Security forces went on high alert and riot police patrolled the streets after several political parties called for rallies to demand the resignation of Mr Bhattarai and protest over his unilateral decision to call elections for November.
Ram Sharan Mahat, a senior leader of the country’s second-largest party, the Nepali Congress, said: “The country has plunged into a serious crisis. This government has no legitimate grounds to continue.”
He said six months would not be enough time to prepare for new polling.
The squabbling political parties in Nepal’s Constituent Assembly failed to agree on a new blueprint for the Himalayan nation by their own deadline of midnight on Sunday, despite repeated extensions of the due date over the past four years.
A key sticking point was whether the country’s states should be redrawn to give regional power bases to ethnic minorities.
Writing the new constitution was supposed to cap an interim period aimed at solidifying details of Nepal’s democracy after the country turned the page on centuries of royal rule and resolved a decade-long Maoist insurgency by bringing the former combatants into the political mainstream.
Mr Bhattarai, from the party of the former Maoists, said the previous constitutional assembly, elected four years ago, had failed and must be dissolved, and that he would head a caretaker government until the November 22 elections.
“We have no other option but to go back to the people and elect a new assembly to write the constitution,” he said in his announcement.
However, his plan drew criticism from legal experts, who said any plans for new polling should be made in consultation with the country’s other political parties.
“It was politically, legally and morally incorrect of the prime minister to announce fresh elections,” said constitutional and legal expert Bhimarjun Acharya.
UN secretary-general Ban Ki-moon expressed disappointment that the assembly expired without producing a constitution. His office issued a statement saying that all parties must “demonstrate the courage and wisdom to come together to address the challenges the nation faces”, and that work on a constitution should resume quickly.
Police spokesman Binod Singh said thousands of police officers had been deployed in the capital, Katmandu, and major cities across the country to stop any violence in the coming days.
At a rally on Monday in Katmandu, small groups of college students burnt effigies of Mr Bhattarai and demanded his resignation. Police quickly put out the flames.
Separately, a group supporting the abolished monarchy also demanded the prime minister’s resignation, blaming him for the country’s political crisis. Police allowed the demonstrators to march through the centre of Katmandu.