Nepal's international donors have accused Maoists of continued extortion, kidnappings and violence against businesses even though the leftists are now members of parliament.
The criticism came from the Industrial Security (IS) group, which is made up of representatives from the embassies of India, France, Germany, the United States, Britain and the European Commission as well as business associations.
"The ... group expresses its deep concern about continued extortion, kidnappings and violence by Maoists against businesses and industries," the group said in a weekend statement.
"Once again, the Maoists are demanding money, lodging and food from the businesses and residences in the Kathmandu valley," the group added, but gave no specific examples.
"If Maoist abuses and trade union militancy continue, Nepal's private sector would be choked and investment will cease," added the statement, issued by the US embassy, the acting chair of the group.
The former rebels' alleged use of violence and intimidation violated a peace deal signed with the government late last year and was "at complete odds with their new status as members of the interim parliament," the statement said.
Human rights groups have also accused the Maoists of continuing abductions.
"Although the cases of abductions and extortion have dropped significantly after the peace deal, it has not completely stopped," said Rupesh Nepal, an official at the Informal Sector Service Centre (INSEC), a leading local rights group.
A total of 232 people have been abducted by the Maoists from various parts of the country, mainly for indoctrination, since the peace accord was signed, Nepal said.
Some have been freed while are still being held by the Maoists, he said.
The Maoists denied the accusations.
"Such allegations have been made to tarnish our image," Maoist leader Dinanath Sharma told the agency on Saturday.
Sharma did say his party was collecting donations from people to organise a massive rally in Kathmandu next Tuesday but added, "We have not forced anybody to provide financial assistance, people are voluntarily contributing."
The Maoist rally will be addressed by party chairman Prachanda, also known as "the fierce one."
February 13 marks the beginning of the Maoist insurgency in 1996 that left at least 13,000 people dead in aid-dependent Nepal, one of the world's poorest nations.
Once foes, the Maoists and mainstream parties have agreed to form a joint temporary government and hold nationwide elections by June for a body that will rewrite Nepal's constitution and decide the fate of the monarchy.
Under the peace deal, the Maoists have agreed to place their weapons and army under UN monitoring. In return, they have been given 83 seats in a new interim 330-seat parliament.