SAO PAULO, Brazil – Banks, schools, and shopping centers shut early in Sao Paulo, buses curtailed service, and callers overwhelmed the mobile phone network as a third day of gang violence spread in South America’s biggest city.

At least 81 people have died since the attacks on police headquarters and cars began over the weekend in Brazil’s financial center, according to Sao Paulo state. Rush-hour traffic clogged main thoroughfares as commuters grew afraid of a new wave of shootings. The federal government offered to send in troops to help restore order.

“I have never seen violence of this intensity in Sao Paulo before,” the president of Sao Paulo state’s Council of Industries, Claudio Vaz, said. Metropolitan Sao Paulo has almost 20 million residents and the surrounding state has a population of 40 million, according to census data.

The 180 attacks were reprisals for the transfer of more than 700 inmates to other prisons in a bid to break up gangs inside the jails, and marked the worst violence in the city’s history. State authorities were also dealing with ongoing uprisings at 45 prisons, in which 180 hostages were being held, after quelling riots at 25 facilities, the state’s Web site said.

“What happened in Sao Paulo was a provocation, a demonstration of organized crime’s strength,” President da Silva told journalists in Brasilia. “It’s very difficult to combat organized crime.”

Nine bus companies, whose fleet of 5,100 buses account for more than a third of the city’s total, halted service today after attacks destroyed 51 buses. The city of Sao Paulo sent two cars with two guards each equipped with bullet proof vests, to protect the city’s 23 bus terminals and stationed guards on main avenues, according to the mayor’s office.

Sao Paulo’s banking union, the country’s biggest union, said 18 branches in Sao Paulo were shut today after being attacked by the criminal gangs.

Most commerce shut down in the southern part of the city on security concerns, according to the city’s press office.

A Catholic university in Sao Paulo, Pontificia Universidade Catolica, canceled its classes today. Sao Paulo’s association of shopping centers said 10 of the city’s biggest shopping centers closed early. Helicopters spotted large groups of workers walking home because of a lack of transport, Globo News reported.

“It’s a very delicate and worrying situation, but it’s still early to imagine a state of general calamity that affects the country’s financial system,” the chief executive of Latin America’s largest bank, Brasilia-based Banco do Brasil, Rossano Maranhao, told reporters at the company’s Sao Paulo headquarters.

Mr. Maranhao said police foiled attempts to attack some of the bank’s branches in the east and south parts of town.

“The situation we’re living in is widespread and not limited to banks, and we’re taking measures to increase security,” Mr. Maranhao said.

On one of the city’s busiest streets, Sao Paulo’s Paulista Avenue, sections in front of police buildings were closed to traffic, in a bid to prevent more attacks.

“My daughter isn’t going to go to school tonight because she’s too scared,” Mara Lucia de Carvalho, 34, said in interview on Paulista Avenue.

Mobile telephone providers in Sao Paulo met with state authorities to consider ways to stop communications in the state’s penitentiaries that is enabling prisoners to coordinate attacks across the city.

Smuggled cellular phones have increased organized crime leaders’ ability to plan attacks and criminal actions from prisons, the head of the Military Police Officers Association, Colonel Luiz Carlos dos Santos, said in a telephone interview from Sao Paulo.

The federal government is prepared to deploy 4,000 national guardsmen to Sao Paulo, as well as army troops, Justice Minister Marcio Thomaz Bastos told reporters in Brasilia after an emergency meeting with Mr. da Silva.