FACTBOX: Key Nepal parties in constituent assembly polls

(Reuters) - Nepalis vote in historic elections on Thursday, their first in nine years, for an assembly meant to write a new constitution and serve as a parliament.

Here are some key political parties among 54 groups contesting the April 10 elections:

* Nepali Congress party -- Nepal's oldest and biggest political party, formed nearly 60 years ago, it has fought for democracy for decades. Has been in power for most of the past 17 years.

In 2006, led mass anti-king protests and forced King Gyanendra to hand power back to political parties. Party headed by Prime Minister Girija Prasad Koirala, 83. Led the peace process with the Maoists ending their decade-long civil war, and has adopted their demand for a republic.

* Communist UML party -- Formed after 1990 by uniting different communist factions in Nepal. A left-of-centre party, wants to turn Nepal into a republic after this week's vote.

* Communist Party of Nepal (Maoist) -- Founded in 1994 by Pushpa Kamal Dahal, widely known as Prachanda. The Maoists began a revolt in 1996 to topple the monarchy.

Signed a peace deal with the government in 2006. Wants Nepal to be a republic and Prachanda to be president.

* Madheshi groups -- Many political groups demand autonomy for the southern plains known as the Madhesh or Terai, bordering India. Biggest among them is the Madheshi People's Rights Forum which organized protests that led to at least 50 deaths in 2007.

* Rastriya Prajatantra Party Nepal (RPP-Nepal) -- Formed by royalists after King Gyanendra lost almost all his powers following the political changes in 2006. The royalists want a referendum to decide the future of the monarchy.

(Compiled by Gopal Sharma; Editing by Simon Denyer)