Guatemala’s congress has chosen a successor for the vice-president who resigned last week amid a widening corruption scandal which threatens to unleash a political crisis in the Central American country.

After a week in which the government struggled to find a candidate the congress would accept, the appointment of constitutional judge Alejandro Maldonado seems unlikely to stall the growing wave of public indignation that helped force Roxana Baldetti out of office.

Baldetti stepped down last Friday after politicians began an investigation into whether to remove her immunity from prosecution. The former vice-president denies any involvement in the corruption scheme, but prosecutors have accused her private secretary of being at the centre of the multimillion-dollar scam.

The scandal has prompted a wave of demonstrations. A fresh protest called for Saturday is expected to be the biggest yet, with activists, emboldened by their success in bringing down Baldetti, now calling for the head of President Otto Pérez Molina.

The crisis is playing out ahead of presidential elections in September, with polls giving a large lead to Manuel Baldizón, a populist rightwing tycoon.

“Baldetti’s resignation came too late to defuse the situation,” said Raquel Zelaya, director of the Asies thinktank and the former head of a body set up to monitor implementation of the peace accords that ended the country’s 36-year civil war in 1996.

Most analysts agree that this is the deepest political crisis of the post-war era inGuatemala. But it remains unclear whether it will eventually strengthen or dangerously undermine the country’s still-feeble democracy.

Zelaya is among the most pessimistic. “We could be heading for chaos,” she said. “If the president is forced to resign there would be a power vacuum and the consequences of that could be terrible.”

Others, however, see a chance for forcing political reforms that could start rooting out the corruption endemic in many Guatemalan institutions.

“The size of the social action shows that people are now really fed up, and they are finally losing their fear,” said Iduvina Hernández, a human rights activist and expert in hidden influence of the country’s military, who supports the call for the president to resign.

“The challenge is to find a common demand that can channel the energy.”

But Hernández warned that the crisis could provoke a repressive response by the country’s authorities – or even a military coup.

The fraud allegations that triggered the current crisis were announced on 16 April by a United Nations-sponsored body of international prosecutors called the International Commission Against Impunity in Guatemala, and known by its Spanish acronym Cicig.

The Cicig’s investigation, now in the Guatemalan courts, alleged the existence of a massive corruption ring that took bribes from importers in exchange from lower customs duties. It charges that the ringleader was Baldetti’s private secretary, Juan Carlos Monzón.

Baldetti is not named in the investigation, but there is widespread scepticism in Guatemala at her claims to have had no idea of what was going on. She was on a trip to South Korea when the investigation was announced, accompanied by Monzón who has not been seen since, though 27 other alleged participants have been arrested.

Baldetti’s position became untenable when the country’s extremely powerful organised private sector added its call on her to resign. She has since been ordered not to leave the country as the investigation continues.

The alleged customs corruption scam is particularly damaging in a country where dismally low tax income is frequently blamed for the failure of efforts to tackle the pervasive poverty.

The customs system also has a long history of association with military-based corruption networks that goes back to the civil war. Monzón is a retired captain, Baldetti’s political rise was reportedly linked to important figures in the military, and President Pérez Molina is a former head of military intelligence.

The scandal has also greatly strengthened the position of the Cicig that was set up in 2007 with a mandate to help dismantle such networks, made all the more complex by the increasing influence in Guatemala in recent years of drug cartels.

The Cicig has conducted a series of major investigations, which have then been channelled through the Guatemalan judicial system, and is credited by many with providing the first signs of hope that that system can eventually become strong enough to stand on its own.

“It made it seem like this could be done,” said Daniel Wilkinson, of the US-based group Human Rights Watch. “It made it seem to the prosecutors, criminal investigators and judges in Guatemala who are committed to this that they are taking risks that are not entirely in vain.”

Wilkinson said that before the customs investigation, Pérez Molina had made it clear he was not intending to extend the Cicig’s mandate. The president, however, announced he would ask the UN to extend the mandate for a further two years a week after the scandal broke.

Analysts say that the demonstrations prompted by the scandal are particularly significant because they mark the first time in decades that the Guatemalan middle classes have taken to the streets en masse to demand political change. The large number of young people, lack of clear leaders and the prominent role of social media in organising the protests has also lent a sense of freshness to the movement.

Saturday’s demonstration is now also set to include a presence of more traditional protest groups, such as poor indigenous farmers. Amilcar Pop, one of the left’s few congressional deputies, said that this development has convinced him that Guatemala is at a watershed.

“I am not so much optimistic as convinced that this is the moment,” he said. “The march on Saturday will be the test of whether this goes to the next chapter or not.”

But the diffuse nature of the protests so far, alongside the focus on the demands for resignations, has some worrying that the protesters could be sabotaging their own cause.

Plaza Pública, a well-respected left-leaning news website, published an editorial this Thursday titled “What do we do now?” in which it argued against pressuring for President Pérez Molina’s resignation. “We need to subjugate catharsis to strategic thinking,” it warned.

With characteristic black Guatemalan humour, the editorial included a video clip from the Life of Brian in which the Judean Peoples’s Front Suicide Squad kill themselves rather than rescue Brian from the cross.