It was chance that made Ana Luque Sillero a native of Madrid and her cousins Catalan. Their parents all left southern Spain decades ago in search of work and a better life for their children, but one couple found jobs in the capital and the others had more luck further north.

No one imagined that the family might one day find themselves facing the possibility of living on different sides of an international border.

“Personally, I feel like it is an attempt to cut off an arm, to amputate something. It is a historic relationship, one of cultural links,” said Luque Sillero. “I myself could have been from Madrid, or Catalan, or Basque. I ended up being from Madrid because my parents settled here. I have cousins with the same Andalucían roots, who grew up in Catalonia and speak Catalan and they are Catalan, but we are one family.”

A leftwing activist who has spent years fighting government austerity projects, she is now supporting the group Recortes Cero in its campaign against the referendum on Catalan independence. So far, their biggest project has been organising a manifesto signed by 2,500 Spaniards from Catalonia and beyond, condemning the referendum as an “anti-democratic fraud”, and running it as a full-page advertisement in the national press.

Catalonia has been plunged into turmoil by the regional government’s controversial decision to call a unilateral referendum on independence for 1 October, and the central government’s heavy-handed efforts to stop it going ahead.

Pro-referendum signs and flags during a Barcelona football match. Photograph: Andreu Dalmau/EPA

The Catalan independence movement has deep roots, and has been fuelled by periods of extreme repression from Madrid, including under the dictator Francisco Franco, who executed the leader of a self-declared Catalan state and tried to suppress its language and culture. Supporters argue that Catalonia has a moral and economic right to self-determination.

Polls suggest that opponents of independence – voters who are happy with the status quo or would like to negotiate greater autonomy inside Spain – make up a slim majority in the region, although they have in the past boycotted unofficial polls on breaking free.

But as Catalans weigh up how to cast their ballot – or whether to vote at all – millions of Spaniards elsewhere, who will be profoundly affected by its consequences, are watching in anger, frustration and sorrow from beyond the regional borders as the bitter battle plays out.

“Quite a lot of people are saying, ‘let them go’, but it will have a brutal cost,” said the Madrid-based film director Fernando Colomo, who began his career in Spanish cinema at a time when Barcelona was a cultural beacon for a country emerging from decades of dictatorship.

An enduring affection for the region, dating back to the Movida cultural renaissance, is evident in his latest film, set in the Catalan town of Badalona. He insists that his objections are to the details of October’s poll, not the principle of self-determination.

“I am in favour of a referendum but one with guarantees, that it’s properly prepared, with information for the citizens,” said Colomo, who was one of the first people to sign the Recortes Cero manifesto. “What they are doing is signing a blank cheque without knowing where it’s going to lead. It is going to be a disaster that will affect all the country.”

Among the complaints on the petition are the lack of a threshold for participation and the pushing through of the referendum in the Catalan parliament in a single day, by a ruling coalition that does not represent a majority of the electorate.

For a country in which dictatorship is a painful living memory, accusations that democracy is being undermined – and that parts of Spanish society are complicit – are extremely powerful.

A giant Sí banner is carried as thousands of people gather for a rally on Catalonia’s national day on 11 September. Photograph: Albert Gea/Reuters

Many leftwing Spaniards are frustrated at seeing the stand-off over the October vote painted as a simple showdown between democrats and authoritarians, and point out that the men at the heart of the current stand-off both lead rightwing parties.

Mariano Rajoy, Spain’s prime minister, and Carles Puigdemont, leader of the Catalan government, have put personal and party politics ahead of national interest, they argue. They worry that the depth of the crisis and the intensity of the debate have sucked attention away from important issues, including the stumbling economy and the impact of austerity policies.

“The idea of this manifesto is to allow us to all express our voice and say, ‘no, I am leftwing, I am progressive, I am not in agreement with Rajoy but I am not in agreement with you [Puigdemont] either’,” said Juan Carlos Torres, also with Recortes Cero. “Puigdemont is dividing the world in two groups, like dictators and autocrats do, saying, ‘if you are not with me, you are against me’, and this is not acceptable.”

There is some support for the Catalan government outside its own region. The radical leftwing party Podemos organised a rally in Madrid last week, in defence of the “right to decide”, after a week in which the central government ordered hundreds more police to the region, seized ballot boxes and arrested 14 senior officials, in a controversial attempt to stop the vote going ahead.

“We are here because in the last days there has been an authoritarian shift, and we think that in response to that we have to show our solidarity with the Catalan people, and defend their right to decide their future freely,” Isabel Serra, a member of the regional parliament, told El Mundo newspaper.

The demonstration drew hundreds of people to the heart of the city, in a rare public reminder of the crisis playing out nearly 400 miles away. Although the referendum is a constant presence in Madrid because of bar talk, TV shows and newspaper front pages, there are few visible signs in the capital of a country in crisis.

Spain’s prime minister, Mariano Rajoy. Photograph: Sergio Perez/Reuters

There are no Catalan flags, no posters for or against the referendum, no appeals to Catalans from their fellow citizens. There is certainly nothing like the unofficial “Let’s Stay Together” campaign launched ahead of Scotland’s independence referendum in 2014. Widely seen as a response to the negative tone of much official campaigning, it drafted celebrities in to “love bomb” Scots, and urged people in Wales, England and Northern Ireland to call family and friends in Scotland and ask them to stay.

For some in Madrid, the debate has dragged on too long, and criticism of the nation’s capital and other parts of Spain has been too intense for much warm feeling to survive. Many are angry or frustrated; others are offended by Catalan nationalists who say that their relatively wealthy region is subsidising their fellow citizens. The slogan “Spain is robbing us” has caused particular offence.

“It hurts, the situation really hurts,” said Maria Jesus Sanz, a 67 year-old retiree. “They are treating us as if we are (worth) less than them.”

Whenever tensions over Catalan independence rise, talk swirls elsewhere in Spain of boycotting Catalonia’s goods or staying away from its beaches or museums, and recent weeks have been no different. “It’s stupid to say, ‘I won’t buy a Catalan product because of this’, but you start to think like that,” said Andres Buendia, a 63-year-old lab technician, who is worried that the vote has unleashed ugly divisions across the country that will be hard to contain.

“These things never stop,” he said. “One of the things we fear is that if this gets worse, people will start asking about your family, about your parents’ surnames [to find out where you are from].”

There is real reason to fear that Catalan independence could fuel far-right sentiment across Spain, one of the few European countries where anti-immigrant populists have not formed a political party, warned Ignacio Molina, senior analyst at the Elcano Royal Institute. It would also be damaging to the economy – Catalonia accounts for 15% of Spain’s population, and 20% of its income – and to the broader project of rebuilding national identity after the restoration of democracy in the 1980s.

“It’s very difficult for the rest of Spain to accept that the Catalans can decide alone about secession, and with good reason,” Molina said. “If we envisage what would happen in Spain if Catalonia becomes independent, it would be a real disaster.”