SAO PAULO, Brazil (AP) – An unprecedented wave of attacks by a notorious drug gang in South America’s largest city entered its fourth day yesterday, with reports of at least 20 more killings that local media said raised the death toll to more than 70.

Masked gang members, apparently enraged at the prison transfer of leaders, hurled grenades at police stations and sprayed them with automatic weapons over the weekend, then turned their rage on the city’s buses on Sunday night and yesterday, torching dozens and stranding tens of thousands of commuters.

Justice Minister Marcio Tomaz Bastos said President Luiz Inacio Lula da Silva was ready to send 4,000 federal police officers to Sao Paulo, but Sao Paulo state Gov Claudi Lembo said the assistance isn’t needed yet. Police said at least 72 people had been arrested.

Silva held an emergency meeting about the violence yesterday as the official death toll stood at 61, but Brazilian media said the total was at least 74.

Authorities say the attacks and uprisings at scores of prisons across the state were orchestrated by the First Capital Command, known by its Portuguese initials PCC. Police say the gang is involved in drug and arms trafficking, kidnappings, bank robberies and extortion rings inside and outside prison.

Many of the new deaths yesterday were prisoners, Brazilian media said. Prison officials said they do not know how many inmates have died.

The killing started Friday night as men attacked bars frequented by officers and police stations, and inmates at dozens of prisons took nearly 200 hostages.

Officials were worried the violence could spread to Rio de Janeiro, where the 40,000 police were put on high alert and extra patrols were dispatched to slums where drug gang leaders live, police spokeswoman Thais Nunes said.

Around Sao Paulo, armed men boarded buses and ordered passengers and drivers off before hurling gasoline bombs inside and leaving them to burn.

Commerce was stifled in Sao Paulo, where millions of people depend on buses to get to work. Thousands of buses never left their garages yesterday. Most stores and businesses opened, but traffic was unusually light on streets and sidewalks.

Weekend attacks killed mostly police officers, as assailants attacked patrol cars, bars where off-duty policemen gather, a courthouse, a highway police outpost and 10 bank branches in poor neighbourhoods. Assailants used guns, shotguns, grenades, machine guns and homemade bombs.

By yesterday morning, uprisings were under way at 45 prisons in Sao Paulo state, Brazil’s most populous. Inmates were holding 196 prison guards hostage.

Enio Lucciola, spokesman for the Sao Paulo State Public Safety Department, said the attacks and prison rebellions “were the most vicious and deadliest attacks on public security forces that have ever taken place in Brazil”.