With Spain's election on Sunday taking place at the height of summer, some 2.6 million voters have opted to vote by post, fuelling fears about possible fraud which have been amplified by the far right.
Holidaymakers who didn't want to miss out had to register early to secure a postal vote, with the authorities receiving 2.6 million such requests -- an unprecedented figure, equating to nearly 7.0 percent of the 37.7 million people eligible to vote.
Such has been the demand that the electoral board said Thursday it had decided "to extend the deadline for voters to hand in their ballot papers to post offices until Friday" in order to make voting "as easy as possible".
The deadline had initially been at the close of play on Thursday.
Since the start of the campaign, the opposition has raised doubts about the ability of Correos, Spain's postal service, to ensure all the ballot papers were counted in time.
"I ask Spain's postal staff to give their utmost.. they are protecting something sacred to the Spanish people: their vote," said opposition leader Alberto Nunez Feijoo last week.
Polls suggest his right-wing Popular Party (PP) will win but without getting an absolute majority.
Santiago Abascal, leader of the far-right Vox which could end up as kingmaker, said Tuesday he was "extremely worried" that Correos "did not have the necessary means" to fulfill the task, denouncing the date of the vote as being chosen "in very bad faith during Spain's holiday period".
- An unknown quantity -
In recent weeks, there has been a flurry of disinformation about postal voting, including suggestions it was expressly chosen to perpetrate "electoral fraud".
"There has never been a (national) vote in summer," said Giselle Garcia Hipola, a political scientist at Granada University who said the unprecedented situation was ripe for disinformation.
"It is easy to spread misinformation because voters have no real perception about how it should work," she told AFP.
In mid-July, Socialist Prime Minister Pedro Sanchez hit out at right-wing insinuations, accusing the opposition of following a strategy "that aimed to create distrust so people won't vote or will be suspicious of the electoral process."
And Correos issued a statement rejecting "all insinuations or reports casting doubt on the work of its professional staff" which "weaken our democracy".
With most voters unfamiliar with the process, some like 51-year-old estate agent Enriqueta Gonzalez struggled with the paperwork.
"The day they announced it on TV, I applied for a postal vote," she told AFP.
But for her it was "very complicated, with too much paperwork", admitting she didn't realize she needed to go into a post office to validate her registration and had simply waited for the papers to arrive -- which they never did.
For others, it was easier than they expected, with 48-year-old Cristina Garcia Loygorri surprised to find she wasn't "waiting in a queue for an hour or two" as she'd feared.
Even so, she had doubts about how it would work.
"I don't really trust it, will my vote really be counted? I'd much prefer to do it in person," she said.
- 'A dangerous strategy' -
According to Astrid Barrio, a political scientist at Valencia University, the main "risk" would be if there was a big difference in the numbers of people who requested a postal vote and the number of those who actually used it.
"That could be interpreted by some as an attempt at electoral manipulation," she said.
International observers have confidence in Spain's voting and postal voting system which is considered as "one of the most solid and reliable in existence", said Joan Botella, a political scientist at Barcelona's Autonomous University.
But when political parties choose to sow doubt, they are following "a dangerous strategy", warned Granada University's Garcia Hipola.
"Once you've sowed doubt about a public body, it doesn't go away," she said.
"It's a typical populist strategy we're seeing all over the world, not just in Spain."