* Workers at 12 refineries still on strike Monday
* Government threatens to use force if blockages continue
(Adds details, background, comments)
PARIS, Oct 18 (Reuters) - A growing number of pumps in French petrol stations were drying up on Monday at the start of a second week of action by refinery and port strikers to block fuel supplies over an unpopular pension reform bill.
Rail workers, truck drivers, high-school students and other protesters also maintained pressure on French President Nicolas Sarkozy in a make-or-break week before Wednesday's deciding Senate vote.
"It's madness. We're submerged," said Paula, manager of a Champs Elysee service station.
Unions called over the weekend to harden the protests, but the government said it would resort to force if necessary to prevent paralysis in the economy.
"When one sees the government's tougher stance, it only reinforces the motivation of the troops," one CGT union official for workers at Total's refineries said.
"As long as the government won't budge, we won't budge," he added.
It was unclear how many of France's 12,000 stations were experiencing shortages.
"The situation is critical," a spokeswoman with Exxon Mobil said. "Anyone looking for diesel in the Paris and Nantes (Western France) regions will have problems," she said.
Nationwide strikes over the pension changes have spread to the country's 12 oil refineries over the past seven days, adding to the impact of a three-week-long strike at France's largest oil port, Fos-Lavera, over working conditions and a port reform.
Refinery and port strikers showed no sign of giving ground, with the prospect of the stoppages continuing at least until a new day of nationwide protests called by unions for Oct. 19.
(Reporting by Muriel Boselli; additional reporting by Valerie Parent, editing by Jane Baird)