By Steve Kingstone BBC correspondent in Sao Paulo

Police in Brazil are under fire, literally - and they have taken to the streets to vent their rage at being victims of violence.

Brazilian police are wary and on the defensive

Ostensibly, Monday's mass rally by police officers in Sao Paulo was for better pay and more resources.

But the reason they say they need those resources is that they are outgunned by their enemies.

They have suffered an unprecedented spate of attacks this past week - all blamed on a notorious criminal gang.

Under attack

Antonio Bandeira of the campaign group Viva Rio offers a familiar critique of Brazil's police: "The problem is very big because the police we have were trained during the years of military dictatorship.

"They are very repressive, not investigative - it's not intelligent police."

The image of corruptible, trigger-happy officers is one that has been reported by the United Nations, Amnesty International and others.

Police are frequently involved in clashes with armed gangs

But over the past week, Brazil has seen a very different side to policing.

In control rooms across the state of Sao Paulo, the airwaves have crackled with reports of nightly attacks on police stations and mobile units.

The assailants have used machine-guns and grenades. Three officers have been killed, 12 others remain seriously injured.

Wilson Morais, president of Sao Paulo's Association of Police Chiefs and Officers, says that in nearly 30 years of policing, he has never known anything like this.

He describes the attacks as "a role reversal" in which the criminals are now hunting down the police.

Lives in danger

To see the practical effect of this wave of attacks, I went to a small police station in the centre of Sao Paulo at about 1030pm.

Whereas there would normally be about three officers on duty here, there were now 10 - heavily armed, standing outside the police station, eyeing suspiciously any car that came past.

Relatives of police killed in action joined Monday's symbolic protest

None of the officers wanted to give an interview. They said their lives and the lives of their families were in too much danger.

But they stressed that the attacks had increased the sense of solidarity amongst police officers and that they would defeat the criminals in the end.

The criminals in question are thought to be members of The First Command of the Capital, an organised crime gang whose senior leaders are already behind bars and now being interrogated.

It is thought they ordered this show of strength, possibly to protest at prison conditions.

Sombre protest

The police are using the attacks to highlight their working conditions.

They want more guns, more manpower and more money - the starting salary for an officer here is the equivalent of just $350 a month.

Alda Carneiro's husband and son were police officers killed in action

On Avenida Paulista, the main thoroughfare through Sao Paulo, thousands of police officers turned out to demonstrate for better pay and conditions.

In the middle was a circle of women sitting around a coffin - all dressed in black, some of them with veils. Behind them were two wreaths of flowers.

These were the widows of police officers who have died in this city in the line of duty.

Little sympathy

Among those who had chosen to join the protest was 60-year-old Alda Carneiro, a widow who said her husband and son had been killed by the criminals.

"We're here today to stand up for all those who are fighting this war - the policemen and their families," she said.

But from most passers-by, there was little sympathy for the police.

One man said: "These days no-one trusts police officers; too many are corrupt. You don't know which side they're on."

"I've got nothing against them personally," said a woman bystander. "It's a difficult time for the police - but they're just not very effective."

Listening to police radio transmissions, you realise that being an officer in Brazil is very different from policing elsewhere.

One broadcast I heard described an incident in which four suspects were killed and six officers were seriously injured.

It's easy to see why Brazilians call it a war.