* French sales tumble 11 percent, tenth monthly decline
* Italian sales down 20 percent
* Fresh falls seen in Spain after uptick (Adds official Italy sales, Italy comment)
By Laurence Frost
PARIS, Sept 3 (Reuters) - Monthly car registrations fell again in France and Italy in August with mass brands suffering most as Mediterranean countries bore the brunt of the euro zone debt crisis and its withering effect on consumer demand.
French sales tumbled 11 percent to 96,115 cars in August, for a tenth monthly decline, the CCFA industry association said, while Italian sales fell 20 percent to 56,447 units, their ninth consecutive double-digit fall, according to automotive think tank Promotor.
The August decline led Promotor to trim its full-year forecast to 1.370 million units from a previous 1.379 million, itself a cut from an outlook of more than 1.4 million units at the end of June.
"Without intervention to help boost the purchasing power of families via tax reductions and a relaunching of consumption it will be very difficult to see any sign of recovery," Jacques Bousquet, President of Italy's car industry association UNRAE, said in a statement on Monday.
Spanish car sales rose 3.4 percent as customers rushed to complete purchases and beat a sales tax rise in September. Analysts said the downtrend would likely resume this month.
"We still do not see any signs of recovery on the horizon," said Flavien Neuvy, head of French auto-industry think tank Cetelem.
The country's main growth drivers are at a standstill, with mass-market carmakers worst hit, Neuvy said. "We are seeing a polarisation of demand, with premium and low-cost brands doing much better."
The mid-market Renault brand saw domestic registrations plummet 30 percent, even as its low-cost Dacia marque recorded a 21 percent gain on runaway sales of its no-frills Sandero subcompact and Duster SUV.
PSA Peugeot Citroen's domestic sales fell in line with the local market's 11 percent drop. Europe's second-largest automaker is cutting more than 10,000 jobs at home as it struggles to stem mounting losses.
Ford, which recently doubled its full-year European loss forecast to $1 billion, saw August sales plunge 17 percent in France and 22 percent in Spain.
French annualised sales are down about 20 percent from their 2010 peak, compared with 35 percent for Italy and Spain, "indicating the potential for further falls in France", Credit Suisse analyst David Arnold said in a note.
Kia was among lower-cost brands that recorded strong increases, as the South Korean automaker and affiliate Hyundai continued to build on a combined European market share that reached 5.9 percent in the first half - up 1.2 points year-on-year.
European No.1 Volkswagen also continued to grab business from rivals. Sales by its namesake brand fell 7.4 percent in France and rose 8.7 percent in Spain, outperforming both markets.
VW's luxury Audi division delivered gains of 8.4 percent in France and 9.7 percent in Spain, as luxury automakers advanced. BMW also rose in both markets.
Fiat brand sales tumbled 34 percent in France and rose 13 percent in Spain.
Spain's market increase was the first in seven months, as customers hurried to showrooms ahead of a Sept. 1 increase in value-added tax (sales tax).
Some 10,000 consumers completed purchases they would otherwise have made later, the ANFAC automakers' association said - setting the scene for a challenging environment later this year.
September and October "are going to be a disaster", Spanish car dealers' association Faconauto said. Sales are set for a sharp fall unless automakers discount even more aggressively, a spokesman said. (Additional reporting by Gilles Guillaume and Elena Berton in Paris, Stephen Jewkes in Milan, and Robert Hetz in Madrid; Editing by Dan Lalor and David Cowell)