Brazil has sent thousands of federal troops to maintain order in the north-eastern state of Bahia after police there went on strike over pay.
Shops and supermarkets in the state capital, Salvador, have been looted and public transport was severely reduced following attacks on some buses.
The Brazilian city is due to host six matches during this year's football World Cup, which begins in June.
Another strike in 2012 led to a spike in murders and other violent crimes.
Back then, more than 130 people were killed and 12 officers were arrested, including the leader of the current labour dispute, Marco Prisco.
He was accused of inciting acts of vandalism by his colleagues, but later released.
The 2012 strike lasted 12 days, and troops were also sent to the streets of Salvador, Brazil's third most populous city.
The police started their latest protest on Tuesday night after failing to reach an agreement with the government in a dispute over higher pay and better conditions.
"If they meet our demands, the strike will end today," announced Mr Prisco.
The government said it was doing everything possible to meet the strikers' demands.
"Our intention is to talk. Firstly, we are concerned about the safety of the population," Bahia's Secretary of Public Safety, Mauricio Barbosa, was quoted as saying by the G1 news portal.
Most schools and universities closed on Wednesday.
Fewer buses were also circulating in Salvador after bus drivers refused to go to work for fear of being attacked.