Bolivia’s presidential election will go to a run-off for the first time, with two rightwing candidates competing for the presidency – marking the end of nearly 20 years of dominance by the leftist Movimiento al Socialismo (Mas).

The candidate with the most votes, however, turned out to be a surprise: centre-right senator Rodrigo Paz Pereira, 57, who had started the campaign with just 3% support in opinion polls.

In second came Jorge “Tuto” Quiroga, 65, a rightwing former president who briefly led the country in 2001 after the resignation of ex-dictator Hugo Banzer.

With just over 92% of ballots counted in the electoral court’s “preliminary” tally, Paz Pereira was on 32.1% and Quiroga on 26.9%.

“I want to thank all the men and women who made this possible and gave a voice to those of us who had none, who didn’t appear in the polls, who didn’t exist,” said Pereira, the son of former president Jaime Paz Zamora, who governed from 1989 to 1993.

Pereira, the senator for Tarija, gave effusive thanks to his running mate, former police captain Edman Lara Montaño, who became known for exposing police corruption and who, according to many analysts, was a decisive draw for voters.

“We will fight corruption head on, dammit!” Pereira shouted to journalists and dozens of supporters waiting for his speech late on Sunday in La Paz.

Quiroga said: “It is a historic night – not for one party, not for one faction, not for one candidacy, but for all Bolivians who have spoken with strength, with faith, with hope and with dignity. Today, we have taken a giant step towards a better tomorrow.”

The electoral court stressed that the figures are “preliminary and not definitive”. That is because Bolivia uses two counts: a quicker one, based on photos of each ballot sent to a data-processing centre, and the slower definitive one, where every vote is publicly counted and scrutinised at polling stations before entering the system.

The court has up to seven days to release the official results.

As neither secured more than 50% of the vote, or at least 40% with a 10-point lead over the runner-up, a second round will be held on 19 October.

Like the first round, the run-off campaign is expected to be dominated by the economic crisis – the worst in four decades – with shortages of dollars and fuel and rising inflation.

Deeply unpopular president Luis Arce, of Mas, chose not to seek re-election and instead put forward his interior minister, 36-year-old Eduardo del Castillo, who won just 3.15% of the vote.

It is a paltry share compared with the more than 50% that had secured first-round victories for Arce and former president Evo Morales in the past – but just enough for the party to avoid losing its legal status, as the threshold is set at 3%.

According to the preliminary count, 19.1% of ballots were null and void – far above the historic average in Bolivian elections, which has typically been below 5%.

Bolivia’s first Indigenous leader, Morales had spent recent weeks urging his supporters to cast null and void votes in protest against rulings by the constitutional and electoral courts that blocked him from seeking a fourth term.

Business tycoon Samuel Doria Medina, 66, who had led polls for much of the campaign, ended up third with 19.89% of the vote.

Doria Medina acknowledged his defeat and announced that he would back Paz Pereira in the run-off.

In the Bolivian press, analysts suggested one possible advantage for Paz Pereira was that the campaign battle in recent weeks had been concentrated between Quiroga, Doria Medina and the left, leaving the senator outside the main line of attacks – or even of fake news campaigns.

Also, polls indicated there were still large numbers of undecided voters before election day.

The highest-placed leftwing candidate was Senator Andrónico Rodríguez, 36, who left Mas to run with a small coalition. Having once polled as high as third, he eventually finished fourth with just over 8%.

More than 2,500 national and international observers, from bodies including the European Union and the Organisation of American States, monitored the vote and were expected to publish their preliminary reports in the coming days. During the day, they said polling had proceeded normally.

According to the electoral court, the election took place without problems, apart from some “isolated incidents.”

One of them involved Rodríguez. As he voted in Entre Ríos, a Morales stronghold about 50 miles from where the former president remains entrenched, the 36-year-old senator was booed and pelted with stones by what he described as “a small group of extremists identified as supporters of Morales.”

Rodríguez had to be escorted by a member of the armed forces to cast his vote. He was not injured. Once seen as Morales’s natural heir due to his Indigenous roots and leadership in the coca growers’ union, the senator was called a traitor for launching his own candidacy.

Wanted on an arrest warrant since October for allegedly fathering a child with a 15-year-old, Morales voted in Villa 14 de Septiembre, about 25 miles from the tiny village where hundreds of coca growers have prevented police and the army from detaining the former president.

Morales denies having committed any crime and claims the case is part of a plan by the current government to destroy him politically.

President Arce, who served as Morales’s finance minister before becoming his main rival, cast his vote in La Paz and said he would ensure “an absolutely democratic transition” in November, when the next president is sworn in.