FACTBOX-Big cities hold key to French vote run-off

(Reuters) - France votes on Sunday in the second and final round of municipal elections, with some of the main prizes still up for grabs.

The Socialists are expected to build on gains they made in the first round last weekend, with administrations run by President Nicolas Sarkozy's centre-right UMP party under threat in several big cities, including Marseille.

Here are some facts and figures for Sunday's vote.

ELECTION PROCEDURE

First-round voting on March 9 gave immediate victory to parties and candidates who received absolute majorities.

Any candidate who took more than 10 percent can go forward to the second round. In practice, many have abandoned the race at this stage and linked up with allied parties that appear better placed for victory.

The candidate who wins a relative majority in the second round is adjudged the overall winner.

The mayors of Paris, Lyon and Marseille are the only ones not elected by direct representation.

Voters in these cities, the largest in France, elect a council for each district, or arrondissement. The councillors subsequently appoint a mayor for the city. Voters know in advance who the various councillors will choose as their leader.