Workers at a factory operated by U.S. firm 3M held their French manager hostage overnight to protest against plans to lay off half the staff.

The industrial director of the group, Luc Rousselet, was barricaded in an office on Tuesday evening and workers are refusing to let him out until he agrees to more favorable terms for the 110 employees who face the axe.

These people have more to complain about than me and I knew there was this risk when I came here, Rousselet told reporters from his office, where the door has been blocked by a wardrobe.

Locking up managers is becoming a tradition in French labor disputes, with police unwilling to intervene to avoid violence.

Earlier this month employees at a Sony factory in southwest France detained the chief executive and human resources director of the Japanese group's French arm overnight and eventually secured improved better terms for workers facing the sack.

Unions at the 3M plant in Pithiviers, near Orleans, south of Paris, are demanding more money for departing staff, guarantees for those remaining and salary payment for those who had gone on strike over the redundancy plan.

Until we have a commitment from 3M that they will increase pay-packets and are ready to discuss our conditions for negotiations, then Mr. Rousselet will have to remain here, union representative Jean-Francois Caparros told Reuters.

If he wants to involve the police, then he will have to send for them, but it is out of question that he leaves without discussing our conditions, he added.

Diversified technology group 3M makes everything from scotch tape to optical films for liquid crystal displays. France is its sixth largest market and it employs 2,800 staff here at eight sites, according to the group's Web site.

The company has said it needs to cut jobs at Pithiviers to reduce overcapacity because of falling demand.

We have had five rounds of negotiations ... and at the end of the last one we reached an agreement on a certain number of points, but there are still measures left where there are substantial points of disagreement, Rousselet said.

Social tensions are steadily rising in France because of the economic crisis. More than two million people are already unemployed and hundreds of thousands more are expected to lose their jobs this year as the recession deepens.

Between 1.2 and 3 million people took to the streets of French cities last week to denounce the government's handling of the crisis and workers from a Continental tire factory facing closure took to the streets of Paris on Wednesday to protest.

(Writing by Crispian Balmer; Editing by Jon Boyle)

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