The incident took place during a tea reception hosted by 57-year-old Prachanda\'s UCPN (Maoist) party to mark the festivals of \'Deepawali\', \'Chhath\' and Nepalese New Year here.
A 25-year-old youth, identified as Pawan Kunwar from Baglung district of western Nepal, slapped Prachanda at the function, according to eye witnesses.
Prachanda\'s spectacles flew off his face after he was slapped, they said.
Soon after, Maoist cadres attacked Kunwar, who got seriously injured, before police took him into custody.
Prachanda, a former Prime Minister, had led a decade-long war in the country against monarchy that claimed 16,000 lives, before the Maoists joined mainstream politics.
Prime Minister Baburam Bhattarai and other senior leaders of various political parties, including Nepali Congress President Sushil Koirala, CPN-UML Chairman Jhalanath Khanal, Madhesi Peoples\' Rights Forum Nepal (MPRFN) President Upendra Yadav, were invited to the tea reception.
On the occasion, leaders from the Nepali Congress, CPN-UML and MPRF-Nepal stressed the need to form a national unity government and forge consensus and collaboration among the parties to conduct fresh elections and promulgate the Constitution at the earliest.
They also slammed the Maoist party and the caretaker government led by Bhattarai for failing to hold fresh elections as per their commitment to resolve the current political crisis.
Nepal has plunged into a deeper political and constitutional crisis as the Constituent Assembly was dissolved in May without promulgating the Constitution.