UPDATE 2-French Sony head reaches deal on plant layoff terms

* Head of France's Sony arm reaches deal with workers

* Agreement on employees over 55, training budget increased (Updates with details, quotes from French Sony head)

BORDEAUX, France, March 13 - The chief executive of Sony's French arm reached a deal on Friday on layoff terms for workers at a plant due for closure next month after being detained there overnight by embittered employees.

Serge Foucher and the Japanese firm's human resources director in France were prevented from leaving the plant late on Thursday and were only released on Friday morning after agreeing to take part in a new round of negotiations.

A representative from the powerful CGT trade union, Patrick Hachaguer, told reporters later in the day that progress had been made in a number of areas, but not on redundancy payments.

"The most important (thing) is that we secured an agreement for employees over 55 years of age," said Hachaguer.

Employees at the Sony plant also obtained an increase in their retraining budget.

Workers locked up the managers in the plant at Pontonx-sur-l'Adour late on Thursday and blocked the road to the site with tree trunks, local authorities said.

Union representatives said their action had been the only way to revive negotiations on layoff packages that were not generous enough.

Foucher told reporters there had been no aggression during his detention. "At no moment did I feel threatened, there was no violence," he said as he left the negotiations.

Tempers have been boiling over in France, hit like other countries by a wave of factory closures and mass layoffs because of the global economic downturn.

Workers in a small northern town hurled eggs and insults at managers on Thursday to protest against the closure of their tyre plant by German car parts group Continental, eliminating 1,120 jobs.

At Pontonx-sur-l'Adour, the Sony plant which employs 311 workers is due to close for good on April 17. Foucher's visit on Thursday was the last before the closure.

Sony had considered converting the plant to make solar panels rather than magnetic components, but abandoned the plan, angering the workers who had hoped to keep their jobs.

Local authorities mediated to avoid a police intervention that would have raised tension further.