RIO DE JANEIRO — The assailants arrived in the sleepy city in southern Brazil ready for battle: They wore head coverings and body armor, and were armed with powerful rifles and loads of explosives.

Shortly before midnight Monday, a series of explosions and volleys of gunfire shattered the calm and jolted the residents awake.

Over nearly two hours, the gunmen held the city of Criciúma hostage, using people as human shields while they detonated explosives at a local branch of the Bank of Brazil, sending a plume of bills flying into the air, and shot at an overwhelmed police force.

“It was a surreal scene,” said Clésio Salvaro, the mayor of the city of about 220,000, said in a televised interview Tuesday morning. “The city was left in a state of panic.”

Brazil has among the highest crime and homicide rates in the world, and bank robberies are hardly uncommon. In recent years, gangs have attempted to dig their way to millions through tunnels, have detonated dynamite to rob banks and have blown up cash machines.

The audacious attack on Criciúma was the latest in Brazil to target a relatively small city with poor defenses, where security analysts say they have good odds of overpowering and dodging security forces that deal mainly with petty crime. Criminals have carried out a handful of similar attacks in such places this year, rather than in the metropolises where large, well-equipped police forces have experience battling violent crime and powerful gangs.

“This tactic is not new in Brazil,” said Samira Bueno, the executive director at the Brazilian Forum of Public Safety, noting there have been a handful of brazen attacks on banks in recent years that appear to be ripped from “the scene of a movie.”

In this latest assault, analysts said a team of roughly 30 gunmen arrived in a convoy of 10 vehicles from outside the state, Santa Catarina, and had planned the operation meticulously. Officials have not said how much money they made off with.

Bueno said it was striking how police officials were unable to stop the attack.

“It’s curious to see how unprepared the security forces were to deal with this incident,” she said. “Clearly it was the work of a group that is highly organized and specialized.”

Elected officials looked weary and shellshocked when they addressed reporters.

“The operation was successful for the criminals, that’s the truth,” Santa Catarina Gov. Carlos Moisés da Silva said. “They pulled off what they set out to do.”

Police officials in Santa Catarina said the gunmen opened fire outside the 9th battalion of the military police, which oversees security in the city, before heading downtown and taking aim at the bank.

The gunfire left a security guard and a police officer wounded. Police officials said the officer underwent surgery and described his condition as “serious.”

Several residents of Criciúma who were shaken from their sleep by the blasts and bursts of gunfire managed to record snippets of the attack from windows and balconies using cellphones.

Some photos and videos showed men sitting in a line along a street. Local news reports described them as a human shield that prevented police vehicles from reaching the bank.