Brazilian police demanding better conditions and more pay went on strike Wednesday, just 37 days before the country hosts the World Cup, threatening to do so again during the football extravaganza.

Federal police walked off the job in several cities hosting World Cup games and in Rio, which hosts seven matches including the July 13 final, they held a peaceful protest just as Brazil coach Luiz Felipe Scolari unveiled his World Cup squad.

In the capital Brasilia -- another city staging games -- striking police surrounded their headquarters and prevented colleagues from entering, Globo's G1 news portal reported.

They also went on strike in several provincial cities including Fortaleza, Natal and Porto Alegre -- all World Cup hosts.

"This is a 24-hour strike which has been well supported," federal police union leader Andre Vax de Mello told AFP.

"We have provided a minimum service for the public, notably at airports, but we are going to stop during the World Cup if the government doesn't provide a satisfactory response," he warned.

"Salaries have been on the slide," de Mello added, saying that of the 13,000 federal police employed countrywide "250 per year leave and take exams to gain entry to other, better-paid public bodies."

The police are paid between 7,500 and 12,800 reais ($3,200 to $5,800) a month before tax and social security contributions.

At Terezina, capital of the northeastern state of Piaui, strikers also gathered outside their headquarters, eyes masked in symbolic protest at what they say is the authorities' inability to improve their conditions, G1 reported.

Last month, troops were forced to patrol the streets of Salvador de Bahia after a wave of looting and murder erupted amid a police strike in the World Cup host city.