A violent police strike in northeastern Brazil that led to a senator being injured by gunfire has shed light on dissatisfaction among officers elsewhere in the country who have threatened to protest as rowdy Carnival celebrations start.

The strike by military police demanding higher salaries in the state of Ceara is a headache for president Jair Bolsonaro, a staunch supporter of police forces who has pledged to curb violent crime.

“Of course, police strikes could spread,” said lawmaker Guilherme da Cunha of the state of Minas Gerais, where police obtained a 42 percent salary increase this year after threatening to strike. “From the moment people who have a monopoly on firearms discover the strength it has, there is a risk.”

In Ceara, violent crime has risen sharply during the police strike, with at least 51 people killed over two days, according to the state's public security secretary. That prompted president Bolsonaro to send hundreds of national guard forces and 2,500 soldiers to maintain order.

During the strike, Senator Cid Gomes was shot in the chest as he tried to drive a backhoe through a police protest. He is in stable condition. Earlier that day, masked officers forced businesses to close, occupied barracks and damaged police vehicles.

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Mayors in several of the state's small cities — 30,000 inhabitants or less — cancelled Carnival celebrations. In Paracuru, where authorities were expecting 40,000 revelers a day, the mayor said he was no longer able to ensure security in his city's streets.

Even though police strikes are illegal in Brazil, other states are at risk of seeing similar protests, lawmakers and public security experts told The Associated Press.

In Alagoas state, civil police, in charge of investigating crimes, have been on strike for two weeks.

“The governor has made a lot of empty promises to the military police. At some point, that bomb can explode,” said lawmaker Davi Maia, who has met police in Congress to discuss their demands.

In Paraiba, military police organized a 12-hour strike on 19 February. In Santa Catarina, public security agents threatened to slow work to a bare minimum, paralyzing operations to an extent but avoiding illegal strikes.

In Rio, one association of municipal guards, who police city parks and properties, began a strike Saturday, during Carnival.

Hooded and masked protesters, supposedly police officers, gather in the 18th Military Police Battalion during the second day of a police strike at the city of Fortaleza in the Ceara state, 20 February 2020 (Jarbas Oliveira/EPA)

Police strikes aren't new, according to Ilona Szabo, co-founder of a security research center, the Igarape Institute. A study by the Federal University of Rio Grande do Sul showed that between 1997 and 2017, Brazil had 715 police strikes, but only 52 by military police.

“More than ever Brazil needs to democratize and professionalize its police forces,” Mr Szabo said.

Many believe police officers are emboldened by the 2018 elections, in which Mr Bolsonaro and other fervent law-and-order supporters were elected. A former army captain, Mr Bolsonaro supported the armed forces during his 30-year legislative career and has said police who kill on duty should be decorated.

Many Brazilians states' finances are in the red, with public servants often receiving partial or delayed salaries. Carnival celebrations often prove a good opportunity for public servants, including police, to pressure authorities, who fear violence and looting during the festivities.

Last year, public security officers in Minas Gerais also chose February to threaten the newly elected administration of Governor Romeu Zema Neto with strikes if he didn't readjust their salary.

“The government was pressured to choose between a terrible, and least worst option,” said state lawmaker Mr da Cunha. Police shut down a motorway and armed men attempted to invade the governor's office, according to witnesses who asked that their names not be used because of safety fears.

As part of the negotiations, the governor obtained an agreement that the increase be postponed one year, meaning the proposal only landed this month in the state's legislative assembly.

The news of a 42 percent salary increase spread rapidly, boosting similar requests in Ceara and other states, and angering governors who have resisted threats of illegal protests.

“Minas Gerais granted this increase, in a state that is not paying salaries, and is in a situation of bankruptcy,” said Ignacio Cano, coordinator of the Violence Analysis Laboratory at the State University of Rio de Janeiro.

“It says a lot about the moment the country is going through, and the strength that public forces are acquiring,” he said.