The first thing I notice wandering through Sao Paulo is the lack of advertising.

There are no billboards, outdoor video screens or ads on buses.

Instead I find tall buildings covered in a mysterious black writing called "pixacao".

In 2006 Sao Paulo's conservative mayor, Gilberto Kassab, passed the "Lei Cidade Limpa" (Clean City Law), in order to combat "visual pollution".

Sao Paulo became the first city outside of the communist world to put into effect a radical ban on outdoor advertising.

Large billboards superimposed over buildings is the regular look of a cityscape to me. After years of trying to ignore such images, my eyes are relieved to see them replaced by an obscure graffiti.

Pixacao is native to Brazil and similar in method to tagging. It's characterised by distinctive large letters painted in a cryptic style. It dominates public space and has created a new context for the everyday life of citizens, forcing new readings upon them.

Born in the early 1980s in Sao Paulo, pixacao was influenced by the political messages written in tar on the streets against the dictatorship of the 1930s-1950s.

The apparent motivation behind those doing pixacao, known as "pixadores", is to make a statement about social injustice, poverty and government corruption.

There is gaping divide between the rich and poor in Brazil and many pixacao crews come from the poorer periphery and are waging war on the elite.

The truth is their work has become a stronger ego expression than political message in most cases these days. Pixadores strive to leave the mark of their crew on every wall possible, competing against one another to reach the highest, esoteric, and remarkable places in the city.

Most Sao Paulo residents hate pixacao and think it makes their city look ugly. I empathise with their anger at its placement. It envelops most of the city, degrading family homes and public monuments. But I can't discredit the typography entirely.

The enlarged letters made with vertical straight lines and sharp endings have a gothic style resembling an ancient Latin script. Pixadores go to great lengths to paint the unusual letters, using themselves as human ladders to scale tall facades. Some have fallen to their death from extreme places.

These writers are not only hostile to the rich and the government. They clash with nearly everyone in Sao Paulo, including street artists.

Travelling along 23 de Maio expressway in Liberdade I find a fantastic mural. It is painted by some of Sao Paulo's finest artists including Os Gemeos, Nunca, Nina, Finok and Zefix. Colourful cartoonish faces watch over eight lanes of traffic. The characters give a bold, bright energy alongside bleak concrete buildings, but a local informs me that pixadores temporarily defaced this work not long after it was completed. Their anger stemmed from the fact public funds were used to commission famous artists to create the mural.

However, there are things the pixadores and street artists have in common. Not least of which is a mutual enemy, the Clean City Law.

Sadly eradicating Sao Paulo of advertising has caused the removal of much of its street art. Many of the images that brought the city an international reputation for artistic innovation have been destroyed. In fact, the original mural along the 23 de Maio expressway was erased by Sao Paulo's clean-up crew back in 2008. This happened around the time that Os Gemeos and Nunca's work was being celebrated at the Tate Modern in London.

The removal of significant internationally recognised artwork caused enormous anger and controversy, prompting the mayor to rethink the law. He created a registry of street art to be preserved and thus sparked a public debate of what constitutes art.

The pixadores however, have never claimed to be artists. With every piece of pixacao removed from walls they retaliate. Armed with rollers and spray cans, they repeatedly paint their script with ferocious speed and strength.

It is a tiresome and expensive battle. Both the pixacao writers and artists continue to dispute this law today. As I walk through the streets, I can almost smell the fresh grey paint used to buff graffiti off walls.

Sao Paulo's current mayor Fernando Haddad is showing signs of change though. In an unprecedented move, he has allowed a four kilometre-long avenue for graffiti. It starts at Patriarca metro station and ends at Itaquera, close to the World Cup stadium hosting the opening match.

Seventy artists have been commissioned and are currently painting the wall with images related to the World Cup and Brazilian football fans. The wall is set to become the biggest outdoor gallery in South America.

It seems likely that pixadores will invade this wall at some point. They have already been plastering their concerns about the World Cup in other parts of the city and they are not the only ones angry.

Thousands of protesters have taken to the streets in recent months unhappy about the amount of public money being spent on the World Cup tournament. They are demanding such things as better health care, education and housing.

Pixacao on the other hand occupies space visually. In a sense their protest, when that's what it is rather than adrenaline-fuelled vandalism, has a louder message.

It's a reminder of the economic inequality, crime, widespread corruption and the poor standards of living that so many of the city's inhabitants endure.

The streets of Sao Paulo tell its own story. There are inspiring chapters, boasting some of the most acclaimed artists of our generation. They speak of beauty, boldness and triumph. Many other walls communicate a darker reality, separating the haves and have-nots inside an immensely creative mecca.

IF YOU GO

GETTING THERE: Qantas flies to Sao Paulo via Santiago (qantas.com.au); Aerolineas Argentinas to Sao Paulo via Buenos Aires (aerolineas.com.ar).

STAYING THERE: Vila Madalena Hostel is a clean stylish hostel located in a safe bohemian neighbourhood (vilamadalenahostel.com.br).

PLAYING THERE: In the Vila Madalena area is a famous alley for street art called Beco de Batman. U Bike offers an architecture and street art cycling tour (u-bike.com.br).

For more on visiting Sao Paulo, check out timeout.com.br/sao-paulo/en

* The writer travelled at her own expense.