Chile, so used to geological upheavals, faces a vastly changed political landscape after a progressive alliance surged ahead in Sunday’s general election, and left conservative presidential frontrunner Sebastián Piñera facing a tough fight in December’s run-off.

Piñera, a billionaire and former president, had been widely expected to cruise to victory – and possibly even win outright in the first round. He still took first place, taking 36% of the vote, but faced a strong challenge by two main leftwing rivals who between them won 43%.

Former TV news anchor Alejandro Guillier, who heads a centre-left alliance, came second in the presidential race, but the real political earthquake, was the emergence of a new political force, the Frente Amplio – or Broad Front – whose roots can be traced to student protests that shook the country in 2011.

Often compared to the Podemos movement in Spain, the FA is an anti-establishment alliance of left-liberal parties, ecologists, humanists and grassroots organizations.

Among the movement’s demands are the replacement of Chile’s neoliberal economic model together with the Pinochet-era constitution; broad changes to the country’s pension system; and major reforms in health, education, workers’ rights and wages.

Led by Beatriz Sánchez, a 46-year-old journalist who came third with 20% of the popular vote, Frente Amplio will now also control 12% of the 155-seat chamber of deputies.

According to political scientist Claudio Fuentes, the main outcome of the election is a fragmented parliament polarised between the right and left – and a complex, unpredictable scenario for the second-round presidential vote in which the FA finds itself a reluctant kingmaker.

“After this triumph by Frente Amplio, the main issue is what proportion of that vote will now go to Alejandro Guillier,” said Fuentes. “Piñera will present a more moderate image to try and win over wavering centrists.”

Frente Amplio supporters now face a choice: do they swing behind a Guillier-led coalition, which includes some of the parties in the current coalition government led by Michelle Bachelet, or do they carry on as radical independents and focus on building a popular base?

Sunday’s election was also marked by a high level of voter abstention, continuing a trend witnessed in presidential elections four years ago when just under half of the electorate turned out to vote.

“The main problem in Chile is a growing disaffection. Many people simply don’t have any interest in or identify with the country’s politicians,” said Claudia Heiss, a political science professor at the University of Chile.

Such frustration with the political class motivated many of the reforms introduced during the present Bachelet government.

One such reform was a gender quota law and women have certainly witnessed a strong improvement in their representation in the country’s parliament. Female deputies in the lower chamber jumped from 16 to 23%, with a similar hike in the country’s senate.

According to Marcela Ríos, National Officer for Democratic Governance at the UNDP in Santiago: “We’ve never had an increase of more than 2 or 3% before in the congress – so this is really significant.”

