When Ada Colau was elected mayor of Barcelona last year, it was a victory for a social movement that has learned how to use social networking to build mass participation. Similar movements in other big cities swept long established politicians out of office, replacing them with newcomers, social activists determined to stamp out corruption and poverty.

Colau’s movement, En Comú (In Common), came out of the local battles against evictions and energy cut-offs that peaked after 2011 as unemployment grew. A loose coalition that includes green socialists and Podemos as well as thousands of non-affiliated people, En Comú and its political supporters has reduced the number of evictions to a trickle and created a broad front to pressurise the government, the banks and the utilities into softening their stance. They’ve also won widespread support for their pro-social housing and fair rent policies at the expense of the established mainstream parties.

To understand how the movement has grown you have to go back to the shock of the Madrid train bombings of 2004. The government run by the Popular Party was forced out of office when its claim that the outrages were the work of ETA was quickly exposed as a lie. It was a turning point in recent Spanish politics, a startling demonstration of the lengths to which the establishment – la casta, as Podemos calls it – would go to protect its position. The story of the years after the economic crash of 2008-9 was the contrast between growing unemployment and mass evictions, as people were no longer able to keep up their mortgage payments, and the endless stories of corruption and pocket lining by politicians of all stripes, and bankers.

The first signs of a change were the occupations in 2011 of public squares in Madrid, Barcelona and other cities by the ‘indignados’, groups of activists and students, in protest against the misery and inequality caused by the economic crisis. The sit-ins fizzled out but there was an intense debate about how to take the movement forward. In 2014, the brutal attempted eviction in Barcelona of local activists from a community centre that the city council planned to demolish to make way for new apartments pinpointed the scarcity of affordable accommodation, the authorities’ disdain for ordinary people. After days of struggle led by neighbourhood associations, the city council backed down but the die was cast and the result was a new local political alliance – En Comú – that contested the general elections later that year, winning the mayoralty and eleven seats on the city council.

There are three key factors in the emergence of En Comú as a political force. First, there is the weight of the neighbourhood associations, a force to be reckoned with in Barcelona since the times of Franco but given a new impetus by the battles over evictions and the cutting off of gas and electricity. Second, there is the inclusivity of the new movement and the determination of activists to avoid the discredited manoeuvring of the mainstream parties. Third, there is the systematic use of social media both as an organising tool – to convene meetings, for example – and to develop and diffuse an alternative political narrative and common sense on issues such as housing and health. Manuel Castells and others have studied the emergence of the new social movements in Spain and the systematic use of social media. He argues that the public space – where ideas are exchanged and dominating narratives are developed – is now less centred on political institutions and more on communication networks. He talks about the power of horizontal communication that enables one person to send a message to ten others – who know and trust her – and for that message to be sent by each recipient to ten more, and so on, creating an instantaneous and massive exchange of information that sidesteps conventional media and governments.

Suddenly, there is the basis for a whole new way of doing politics. So, around 6000 people have been involved in the discussions about housing policy. Altogether, around 40,000 people are registered supporters with four times that many voting for En Comú. An estimated 1700 are the active core with a ‘central committee’ of forty, twenty three of them women. Policy is decided by commissions, for issues and areas, using face to face events and social media, especially telegram, a more secure alternative to what’s app. All this provides the means for a on-going conversation between horizontal power – the citizens and their networks – and the vertical power of the mayor and her cabinet.

What marks out the new political approach, above all, is the commitment to mass participation in decision making, to bringing together different forces under the same umbrella (‘confluence’), united on the basis of broad principles about how to do politics rather than detailed manifestos, and the massive involvement of women at every level. Not everyone who supports En Comú favours Catalan independence, but the movement’s line – that the Catalans should have the right to decide for themselves – unites all but the most hard line supporters of the government. After the indecisive outcome of last year’s general election and the months of frustrating political horse trading that followed, Spain faces new elections on 26 June. This will be a big test for En Comú and its allies across the country: can they maintain their momentum and can they unite horizontal people power with vertical state power so as to play a key role in deciding the future shape of Spanish government.