Police officers detained a high school student during a demonstration against retirement reforms in Lyon, France, on Friday. French students intensified blockades of high schools and universities Friday. LAURENT CIPRIANI / THE ASSOCIATED PRESS

PARIS -Protests and industrial action against planned pension reforms in France continued for a fourth day Friday as fears about fuel supplies intensified and students took to the streets again.

Production at the country's 12 crude refineries remained stalled by striking workers, according to union officials, and the situation at oil storage depots away from the refineries was tense.

There were also concerns about a possible shortage of jet fuel at the main Paris airports after news reports had quoted Trapil, a supply company, as saying that pipelines carrying jet fuel to the airports had stopped operating because of industrial action. Trapil did not respond to calls for comment, but a spokeswoman for the airports said there was enough fuel for several days.

Strikers and union members moved late Thursday and Friday to block supplies from leaving a number of storage sites, while police intervened at several sites to ensure that supplies could leave, according to news reports.

In interviews on French radio, Dominique Bussereau, the secretary of state for transport, justified the use of the police. "We can't allow a shortage of fuel," he said, adding there would not be any supply problems this week.

Overall, the action appears to be reaching an important phase. The largest labor unions have called for national demonstrations today and truckers have vowed to block some freeways this weekend.

In Paris, buses and Metro trains were back running at almost normal service Friday, but commuter trains and the national rail network remained disrupted. Turnout among striking workers at the rail operator SNCF dropped to 15 percent on Friday, a steady decline from earlier in the week.

President Nicolas Sarkozy showed no sign of backing down. He said Friday that the pension changes were aimed at "social justice" and added, "as head of state, it's my duty to look to the future."

The changes include raising the age of retirement with a full pension to 67 from 65 and increasing the minimum legal retirement age to 62 from 60.