FACTBOX - How do Spain's parliamentary elections work?

(Reuters) - Spain is voting on Sunday to elect a new Congress and Senate.

Here are some facts about the electoral system:

PARLIAMENT: Made up of a lower house, the Congress, and an upper house, the Senate. Laws are presented and debated in the Congress before passing to the Senate. The Senate may propose amendments and even veto legislation. However, Congress can override a veto immediately through an absolute majority vote, or by a simple majority vote after two months.

All 350 Congress deputies are directly elected for four years.

In the Senate, 208 members are directly elected, also for four years, and another 56 -- increased from 51 in the last legislature -- are appointed by Spain's autonomous regions.

2004-2008: At the last elections in 2004, the Socialist Party (PSOE), headed by current prime minister Jose Luis Rodriguez Zapatero, won 164 seats in the Congress. The Popular Party (PP), led by Mariano Rajoy, won 148. Ten seats went to the Catalan party CiU. Esquerra Republicana (the Catalan separatist Republican Left) won eight seats, the Basque Nationalist Party (PNV) seven and United Left five. The others went to smaller groupings.

Zapatero ruled without an absolute majority in either house, securing votes by means of ad-hoc coalitions with leftists or regional parties.

TIMETABLE: The newly elected chambers convene on April 3 after King Juan Carlos has held consultations with the leaders of the main parties and proposed a candidate for the post of prime minister.

WHO VOTES: Some 35 million Spaniards are eligible to vote, 1.2 million of whom live abroad.