* Turnout key after days of anti-reform strikes
* Pensions reform a key test for Sarkozy
By Catherine Bremer
PARIS, Oct 16 (Reuters) - France's powerful unions hope to mobilise millions of marchers on Saturday to take to the streets in a nationwide protest against President Nicolas Sarkozy's flagship plan to raise the retirement age.
The country has already endured four straight days of strikes over the government's unpopular pension reform, squeezing fuel supplies, grounding flights and cutting rail services.
The government is determined to push ahead with the legislation, saying it is the only way to a stop a 32 billion euro annual pension shortfall ballooning to 50 billion by 2020.
Union leaders want to bring enough people onto the nation's streets to make the government think again, while ongoing strikes mean France will continue to be hit by travel disruption at the weekend.
The government and unions will be looking at the turnout for indications about the strength of the protest. Unions said demonstrations on Tuesday drew 3.5 million people onto the streets -- the interior ministry put the figure at 1.23 million.
Rail networks on Saturday will be affected as may air travel
after runway worker strikes at Paris's Orly airport on Friday
grounded some Air France
The government's main worry for the weekend is students becoming rowdy. Riot police used tear gas on Friday to disperse crowds in the city of Lyon. Dozens of students were arrested across the country and several police were injured.
In another sign of growing momentum to stop the minimum retirement age rising to 62 from 60, truck drivers -- the heavyweights of French demonstrations because of their ability to block roads -- have also heeded a call to join the action.
Maxime Dumont, head of the CFDT union's trucking section, said drivers could block fuel depots, refineries and food warehouses and clog roads by driving slowly along them.
"In the transport sector we can do a bit more to help the workers. We are going to join the movement to make the government give way," Dumont told Reuters.
France has a long tradition of overpowering unpopular government proposals through militancy on the street, although analysts believe many French people are reluctantly coming to terms with the fact that raising the retirement age in line with other European countries is inevitable.
Strikes during the week at French oil ports and all the country's 12 refineries put pressure on petrol and diesel supplies across the country, notably when supply was cut into the Paris region and international airports on Friday from a pipeline running south from Le Havre. [ID:nLDE69E0D3]
Turnout at a nationwide rail strike slipped to 15 percent by Friday, however, from 40 percent on Tuesday. Police were also able to break up blockades at three fuel depots in southern France, easing some of the pressure at petrol stations.
In Paris, demonstrators will meet at the Place de la Republique square from early on Saturday afternoon and march to the historic Bastille square, led by trade union leaders. (Editing by Matthew Jones and Jon Boyle)