The former fighters, now the backbone of Nepal's new government, have repeatedly called for an end to Nepal's 239-year-old monarchy.
Other major parties have joined the Maoists calls for the king to step down.
"The first meeting of the constituent assembly will definitely end the monarchy and there will not be any compromise on this," Prachanda, the Maoist leader, said after meeting with UN officials and foreign ambassadors in Kathmandu.
Seeking US support
He also appealed to Washington to remove them from the US list of organisations the US administration considers "terrorists".
- Tucked in the Himalayas between China and India, Nepal occupies an area of 147,000 sq km, with a population of 26.4 million
- Nearly one-third of its people live on an income of less than a dollar a day
- Eighty per cent of Nepalis are Hindus
- Nepal was the world's last Hindu kingdom, before declaring itself officially secular in 2006
"We want to establish new diplomatic relationships with the US government and get support from them to build a new Nepal.
"I hope the US will reconsider their policy," Prachanda said.
But the Maoist leader said he could not rule out the use of violence in the future.
"Right now, I cannot renounce every kind of violence," he said.
"We want to lead this process to a logical conclusion and we want to create a model of peace. Through this we want to renounce reactionary violence."
The Maoists fought against the government in Nepal's civil war, which ended in 2006 and saw 13,000 people killed.
In recent days, the Maoists have been in talks with the other major parties about forming an administration and are pushing for the creation of a president, a job they want filled by Prachanda, their leader.
"It is my desire to be the president," Prachanda said on Thursday.
"But since there is no provision in the present constitution, we will have to reach some agreement with the other political parties."
Preliminary results show the Maoists' Communist Party of Nepal have won a total of 217 seats in the new 601-member constituent assembly.
In second place is Nepal's traditional electoral power, the Nepali Congress, with 107 seats.
The other major party, the Communist Party of Nepal (United Marxist-Lenninist), which had been expected to win the election, came in third.
The assembly, which is expected to sit for the first time in coming weeks, is charged with rewriting Nepal's constitution while it governs the country.
King Gyanendra, the Nepali monarch, was forced to end a short period of direct rule in April 2006 after mass protests by the Maoists and mainstream parties.
Gyanendra began his reign in 2001, after nine members of the family of the previous king were killed, reportedly by the crown prince who then shot himself.