At least 200,000 people have taken part in a Maoist rally in the Nepalese capital, Kathmandu - the first there by the rebels for three years.
Streets were hung with banners bearing communist slogans and posters of the Maoist leader, Prachanda.
King Gyanendra ended 14 months of absolute rule in April after weeks of pro-democracy protests.
The new multi-party government has been holding talks with the Maoists aimed at ending the 10-year insurgency.
The Maoists are demanding the dissolution of the government and elections to a new constituent assembly.
The BBC's Charles Haviland in Kathmandu says Friday's rally was huge, with people crushed against fences and climbing walls for a better view.
It featured folk music and dance displays, characteristic of the Maoists' rural rallies.
Hundreds of youths in T-shirts featuring the Maoist leader Prachanda kept the crowd orderly.
Hundreds of public transport vehicles were allegedly seized from villages around central Nepal and people compelled to come.
"Get rid of the royal regime... We want a republic state," they chanted as they arrived for the rally.
Others told the BBC they had turned up because they wanted to hear the Maoists' social message.
Some city hotels said they had been pressurised to let out rooms at reduced rates, or that young cadres had been putting up tents in their grounds.
The government tightened security in sensitive areas, and asked the demonstrators not to march towards the royal palace.
The Maoists have been in talks with the new government
Rebel negotiator Krishna Bahadur Mahara addressed the crowd and said the government had been slow to implement a move to hold elections to a constituent assembly.
"The present parliament is incapable of representing people's voice so it should be immediately dissolved," he was quoted as saying by AFP.
The government and the rebels have recently agreed to continue the peace process and follow an agreed code of conduct.
However, the rebels have already been accused of violating the code.
A Maoist leader has admitted the rebels killed two men in southern Nepal after kidnapping them and bombing their parents' house.
Coinciding with the rally, King Gyanendra is due to make his first public appearance since he gave up direct rule.
The king will attend a religious ceremony in nearby Lalitpur district, less than 10km (six miles) from the Maoist rally.
More than 13,000 people have been killed in the 10-year insurgency.