There is thick black paint on virtually every wall or facade here. "What's with all the graffiti?" - you might ask.- "It's not graffiti, it's pixação."

At first sight, it is difficult to tell the two styles apart, but there are important differences. In the case of graffiti – be it tagging or bombing – the letters are rounder and more stylish thanks to the copious use of blending, shading and other techniques. Colour is another important element: the brighter the better in most cases, in images and figures too.

By contrast pixadores, as practitioners are called, (sometimes spelled pichadores) seldom create visuals, only letters. Their ubiquitous calligraphy is composed of straight lines and sharp edges, giving their creations – pixos – a jagged look. They are also primarily black (the verb "pichar" in Portuguese means to cover with tar). But just because pixos are monochromatic and less stylised does not mean they lack history or socio-cultural significance.

The use of São Paulo's city walls as a canvas is not new. In the 1930s, political candidates wrote campaign slogans all over them. By the late 1960s, when students took to the streets to voice their dissent against Brazil's military government, spray painting phrases such as "abaixo a ditatura" ("down with the dictatorship") on the walls of public buildings became an important act of protest.

The style we now identify with pixação first emerged in São Paulo in the 1980s. Politically, the country was undergoing a gradual transition to democracy, but politics weren't the only thing on the mind of São Paulo's youth – so was heavy metal.

The musical genre that developed in the UK and US gained a strong following in São Paulo. In addition to the brute force of bands such as Iron Maiden, Judas Priest, AC/DC and Metallica, Paulistano youths were also attracted to their album covers – in particular, the runic-inspired typeface these bands used to brand themselves.

In true Brazilian fashion, youths in São Paulo cannibalised this foreign practice. Thus began the evolution of this city's distinctive pixação: a style of urban writing that has inspired numerous pixadores to come up with their own variations on this type of calligraphy – according to one estimate, there are more than 5,000 active pixadores in São Paulo alone.

At its most basic level, pixação is about vanity, fame and self-promotion, which is why the vast majority of pixos are either personal monikers or the names of particular griffes (collectives). Fame in the world of pixação is primarily a numbers game – so much so that seasoned pixadores boast about having left their mark on nearly every wall of the city. Currently, one of São Paulo's most famous and prolific pixadores goes by the moniker RAPDOS, a variation on the word rápido.

Pixação is also about visibility, particularly the kind that can only be achieved through daring acts of courage. In its most basic form the targets are walls and the risk is relatively low – although it is still a criminal offence that carries a potential prison sentence.

The more extreme form is janela de prédio ("building window"), for which success is measured in terms of height. Pixadores – usually in teams of two – climb a building's facade by grabbing on to its window ledges and pulling themselves up, floor by floor, leaving their pixos as they go up. Rooftop pixos require guts and the right equipment – black ink and a paint roller attached to a broomstick – but sometimes that's not enough, and to extend their reach, pixadores have to dangle their bodies over the roof ledge.

These daring acts, however, do not come close to escalada, or "buildering", whereby pixadores scale the outside of a building by holding on to its external surge arrester cable. This is a particularly perilous way to climb a building considering that the clamps used to fix this cable onto the facade are not built to withstand the weight of a person. To make matters worse, escaladas are executed at night by alone pixador.

Not surprisingly, accidents are common – and sometimes fatal. But for the adrenaline-seeking pixador, the pay-off is worth the risk. By scaling the building in such a way, they can access large sections of a facade that have never been touched by another pixador. Unlike graffiti (which many pixadores reject as being "too commercial" and a "beautification scheme"), pixação seeks to positively degrade the urban environment. As one pixador put it, pixação is "an assault on the city".

This hostile relationship is ingrained in the very language of pixação. For instance, pixadores never use the term "paint" or "spray". Instead, they prefer "arrebentar", "detonar" or "escancarar" ("smash", "blow-up" and "destroy"). Some typical pixador monikers translate as "shock", "neurosis", "death", "scare", "nightmare", "danger" and "nocturnal attack".

This anger towards the city is much more than teenage bravado or youthful rage. It is rooted in a sense of social injustice that is intrinsically connected with the pattern of uneven urbanisation that began in the 1940s and continues today. Seeking to remake São Paulo into a modern city, elite reformers and boosters of the 1940s and 50s embarked on ambitious urban renewal projects. In addition to infrastructural improvements, a street widening programme, the construction of a massive urban park (Parque Ibirapuera) and other beautification projects, the main feature of São Paulo's urban renewal was its modernist skyscrapers.

Fuelled by easy credit, ambitious developers and aspirations for a New York-style skyline, São Paulo experienced an unprecedented building boom in the immediate postwar period. Some of the city's best known modernist buildings date back to this period, including David Libeskind's Conjunto Nacional, Franz Heep's Edifício Itália, and Oscar's Niemeyer's iconic S-shaped Copan building.

But while such urban renewal projects may have benefited better-off Paulistanos who lived and worked in and around downtown São Paulo, they had an adverse effect on the lives of the city's working-class residents. To transform São Paulo into the modern city envisioned, large portions were demolished, especially the "outmoded" buildings located in the downtown area inhabited by the working poor class. Unable to find affordable housing in and around downtown, working-class Paulistanos were left with two bad options: join the urban poor in one of the city's growing favelas, or relocate to the periphery. Most chose the periphery.

Life there was, and still is, challenging. Far from São Paulo's downtown area where most jobs are concentrated, peripheral neighbourhoods also lacked the basic public services associated with modern urban living, including a proper sewage system, running water, paved roads, electricity, hospitals and schools. As a result, São Paulo earned the reputation of being one of the world's most unequal cities, divided between the haves of the centre and the have-nots of the periphery.

The anger that pixadores felt – and still feel – towards the city should be understood in the context of this uneven pattern of urban development. In the words of a well-known pixador, "Pixação is a reflection of the absence of the state in the life of that person who decided to become a pixador." It is no coincidence that the vast majority of pixadores hail from São Paulo's peripheral neighbourhoods and, just as important, that their preferred targets tend to be the centrally located modernist buildings – especially those designed by famous architects.

In recent years, pixadores have targeted icons of São Paulo's modernism, including the Wilton Paes de Almeida building and Niemeyer's famous pavilion located inside Ibirapuera Park. Pixadores have also tarnished sites that are part of the city's historic patrimony, including the Ramos de Azevedo fountain in downtown São Paulo. The more sacred the site, the more attractive it is as a target for their pixos.

Many pixadores approach their craft in terms of politics. As one pixador put it in a recent documentary by João Wainer, "We practise class warfare." Others are more romantic, hoping that their pixos, by tarnishing the appearance of the more privileged areas of the city, will encourage better-off Paulistanos to reflect on the way working-class residents live – especially those in the periphery.

Unsurprisingly, however, the more common reaction to a wall full of pixos is resentment. To city officials and the "victims" of pixação, pixadores are vandals whose creations – which one observer referred to as "an urban plague" – must be eradicated at all cost.

Local authorities and residents have been engaged in a battle to stem the flow of pixação since the early 1980s, when the practice first emerged. Yet despite hi-tech security cameras, neighbourhood watch groups, police intimidation, draconian laws and a special sanitary unit within the city government dedicated to covering up pixos, pixação is more popular and widespread in São Paulo today than ever before.

As any pixador will tell you, illegality is what keeps pixação "fresh" – and what distinguishes it from graffiti (which is not consistently illegal). In recent years, as graffiti artists from São Paulo such as Os Gemeos have begun to attract international notoriety – with major exhibitions at the Tate Gallery in London and other venues – politicians in São Paulo have become more tolerant of graffiti, and even proud of its local roots. Pixação, on the other hand, remains entirely illegal and pixadores continue to rodar (slang for going to jail).

The city's authorities may be no match for pixação, but there are signs that the market forces that it could be transformed it into an "acceptable" urban expression now hope to do the same with pixação. Pixação – and the image of the pixador as a subversive figure – has already been appropriated by such international brands as Puma to sell their apparel. A pixação-inspired font, Adrenalina, can be downloaded for US$25 and, in 2012, the 7th Berlin Biennale for Contemporary Art invited a group of pixadores to make an "artistic intervention".

While some pixadores have embraced the idea of marketing pixação as a "legitimate" art form, others are much more critical. In recent years, one group has invaded a number of art galleries in São Paulo's hippest neighbourhoods that were exhibiting (and selling) works by pixadores and photographs of pixos. In an act of protest, they covered all the pieces with black ink and painted slogans such as "sell-out" and "the street does not need you". For these pixadores, ensuring that pixação remains a marginal expression of the urban periphery – as opposed to a marketable commodity – is essential to its very survival.