The elected assembly will rewrite the constitution and the Maoists have asserted that their first action will be to make the country a republic.
Bhattarai said that the elimination of the 240-year-old royal court was unstoppable.
"In the first meeting of the constituent assembly we will declare the country a republic, then we will notify the king to leave the palace.
"As an ordinary citizen he will have to abide by the law."
The election uses a first-past-the-post system for 240 seats and proportional representation for 335 seats. A new cabinet will appoint 26 members.
Officials said that the Maoists lead in the proportional representation tallies, so far claiming 31 per cent of the counted vote, about 40 per cent of the total vote.
The Nepali Congress is their closest opponent with 23 per cent.
Dilliram Bastola, an election official, said: "The Maoists have the highest percentage of votes so far, but we cannot know the number of seats they will be allocated until we have counted all the votes."
Proportional representation votes are due to be counted in full by next week.
The elections were a key factor in a 2006 peace deal which ended the Maoists' 10-year battle and brought them into mainstream politics.
In an effort to establish a communist republic, the Maoist fight to take down the monarchy left at least 13,000 people dead.
The king took power in 2001 after eight members of the royal family were killed by Crown Prince Dipendra, who also shot himself.
Gyanendra then took absolute power in 2005 to fight the Maoists.
Some politicians have said Nepal should keep a monarch as a national symbol of neutrality between China and India.
Bhatterai has said that any coalition government the Maoists form will include mainstream parties.
He said: "We are confident we can work with other parties. There is no alternative as this was not a general election - it was an election to make a new constitution."