At least 52 people have died in two nights of violence in Sao Paulo state, Brazil, with criminal gangs attacking police stations and riots in prisons.
State officials say the unrest is being organised by the First Command of the Capital (PCC) criminal faction.
There have been revolts in more than 70 state jails and more than 250 people are being held hostage.
But State Governor Claudio Lembo said the situation was under control, and rejected a federal offer of help.
He said there was no need for extra police, or for troops.
Those being held hostage in the prisons include inmates' families, many of whom had been visiting on what was Mothers' Day in Brazil.
The 52 dead include 35 police officers and prison guards.
The BBC's Steve Kingstone in Sao Paulo says the mood in the state police force is extremely dark and tense.
Bigger police bases have extra security, he says, and nearby roads are coned off to prevent drive-by shootings.
Smaller mobile units and street corner booths have been closed down.
The transfer of about 600 prisoners to a maximum security unit is thought to have sparked the unrest, which raged throughout Friday night.
Officers in police stations, mobile units, at their homes or in bars were targeted.
Violence erupted again on Saturday evening, with police patrols and off-duty officers again targeted with grenades and machine guns.
In one incident, dynamite was placed at the entrance to a police station and the resulting explosion blew off doors and iron railings.
Our correspondent says that, taken together, the attacks and the prison revolts represent the biggest wave of organised violence in Sao Paulo's history.
Founded in 1993, the PCC has been involved in drugs and arms trafficking, kidnappings, bank robberies, and prison breaks and rebellions, police say.
In November 2003, the gang attacked more than 50 police stations, killing three police officers and wounding 12.
Those attacks were thought to have been orchestrated by PCC leaders in jail.