Influence comes from behind prison bars

The authorities say the shootings may have been orchestrated by imprisoned drug ring leaders in retaliation for a foiled prison break on Sunday.

In Tremembe, about 150 kilometres (90 miles) north-east of Sao Paulo, a convict was killed by guards when 28 inmates tried to escape from a prison.

Authorities have said they believe the escape attempt was aimed at freeing a jewel thief.

It seems like it's organised crime trying to show us they are still out there Saulo de Abreu

Eight police stations were attacked throughout Sunday and into Monday.

"It seems like it's organised crime trying to show us they are still out there," said Saulo de Abreu, chief of Sao Paulo state police.

He denied the sprawling city of 18 million is becoming like Rio de Janeiro - where police and drug dealers frequently engage in what often appears to amount to urban warfare.

Take-away ban

There have been previous bouts of violence apparently on the orders of imprisoned criminals.

Rio's most infamous drug lord allegedly oversaw from his prison cell a wave of violence before the opening of the carnival.

Reports said Luiz Fernando da Costa was upset about being stripped of prison privileges, including conjugal visits and take-away restaurant food.