The unknown attackers drove the lorries down an embankment causing them to overturn. The drivers were unharmed, but ten of the pigs were killed.

The incident on a country road 50km (30 miles) south of Lisbon follows violent protests by Portuguese pig farmers against cheap imports from Spain and other countries.

Although no-one has claimed responsibility for the attack, Portuguese farmers are strongly suspected.

Sign of desperation

A spokesman for the local pig farmers' association, Jose Manuel Pinto, said it was a sign of desperation.

"When someone is in despair, he does things that in normal circumstances he would not do," he said.

President Jorge Sampaio and Portuguese government ministers have criticised the action as totally unacceptable and called for a thorough police investigation.

In recent weeks, pig farmers have released pigs onto the streets to draw attention to their plight.

Last week hundreds of them clashed with police in central Portugal and five people were slightly injured.