The seats in the house began to fill just before kick-off on the opening day. The party had started early and it continued below, where the band played on, but it was evening now and it was the team’s turn so the inhabitants of Indalezio Ojanguren Street No1 and No2 hurried home for a better look. On the side of the tallest buildings in the smallest town the first division has even seen, balconies filled. Thirteen floors where red and blue flags hung. Some carried the slogan: “Primeran Ikusten Zaitut Eibar.” Loosely, “We can see you in Primera, Eibar. Below them, the Estadio Municipal de Ipurúa filled too. It was starting to feel like the whole town was there. Well, not the whole town, but not far off. There are 27,378 people in Eibar, in the valley of the Ego, where escalators carry you up steep streets. There were 5,173 of them were inside the Ipurúa on Sunday afternoon, with its squat, square stands tucked into the hillside, overlooking the town.

Another 130 or so watched from the flats. They didn’t want to miss this. For the first time in their 74-year history, SD Eibar were about to play in the first division. They had never expected to be there and when they got there it was almost taken away from them, another victim of a rotten system. But Eibar had overcome. And they were about to do so again. Eibar had only even been close once before, a decade ago. Back then, Gaizka Garitano played for them.

Now, aged just 39, he is their manager. Fourteen months ago, his team played in Spain’s regionalised, four-division, 80-team Segunda División B. Two promotions in a row took them to La Liga. When they kicked off on Sunday, eight of the XI had played in the second division with them last year; seven of them were survivors of the Segunda División B.

Only Ángel had ever played in the top flight before. No town this small has ever been there. No team this small has either. Last season, Eibar’s budget was just €3.2m; this year, their entire budget couldn’t cover Cristiano Ronaldo’s or Leo Messi’s wages. They have been built on loan deals, players coming and going every year. Traditionally, Eibar was seen as a place where young players toughened up: David Silva had played here, Xabi Alonso too.

Their sponsor is a local scrap metal business that first appeared on the shirt when no one else wanted to. There are 4,000 season-ticket holders and 600 more on the waiting list. You could get the population of Eibar into the Camp Nou and still have room for 71,000 people. A 15-floor block of flats would tower over most stadiums, sure, but 14 of the 15 floors in the two blocks by the ground overlook Ipurúa. The stand in front of them goes back just five rows. Temporary seats increased the capacity – by less than 700 – and Ipurúa is smaller than the 6000 minimum required in the Segunda División, let alone the 15,000 for La Liga.

They have a year to remedy that, compliance has not always been enforced (Numancia only had a capacity of 9,000) and expansion would be pointless: who’s going to sit there? On Sunday, a handful of the seats in the temporary stand remained empty. But being small was a problem before and it could be again. As Eibar headed towards the Segunda División title, they found themselves in trouble. A “model club”, in the words of the league’s president, they had no debt. But according to the law that obliged all clubs except Madrid, Barcelona, Athletic and Osasuna to become SADs (PLCs) in 1992, their social capital, their value in shares, had to be at least 25% of the average expenses of all the Segunda División teams, minus the two biggest and the two smallest. They had to take their social capital to €2,146,525m, raising over €1.7m in share issues. If not, through no fault of their own, they wouldn’t just be denied promotion to the first division but would be relegated to the Second Division B. They’d go down two divisions even more quickly than they had come up.

A campaign began. Fans bought shares; shops became stakeholders. No one could buy more than €100,000’s worth: they did not want to fall prey to speculators. Xabi Alonso and Asier Illarramendi were among those supporting it. Amazingly, it worked. Luis Maria Cendoya, a 90-year-old who has been a member since 1945, bought the share that saw them go over the threshold. On the main wall of the ground, the names of shareholders are going up in gratitude. There are lots of Basque names, but look carefully: “Paul Reidy, Ireland.” 10,000 people bought shares in 50 countries. Before kick-off, a message went round the perimeter in a series of languages: thank you.

And so here they were, where they never imagined they’d be. “If I’d said so, you’d call me a lunatic,” wrote one veteran reporter. It could not have been more perfect. Their first game was against Real Sociedad, the other team from the province of Guipuzcoa – the Primera División team many Eibar fans “support” after their own. Support-ed. It felt right somehow. At kick-off on Sunday, all 22 players were Spanish, the first time that had happened in four years. Thirteen of them were Basques, nine of them from Guipuzcoa. In the Real Sociedad starting XI, four had played for Eibar. Yuri Berchiche was there last year, on loan.

Fans sang together and drank together. They began filling the Plaza Unzaga early, to the sound of drums and chants, surrounded by shop windows with the Eibar shield and scarves and flags. Everyone seemed to be wearing a shirt. The Eskozia la Brava supporters club, whose Scotland flag is painted on the white wall of the ground, set up a barbecue. Fireworks strung across the square exploded and filled the air with smoke. Street performers put on a show. Bars were packed, spilling out onto the square through the arches. Everyone seemed to be wearing shirts; every window seemed to bear a flag. Patxaran was poured. Sitting on one of the terraces, watching it all unfold was Eibar’s new winger, Dani Nieto.

The afternoon wore on and the game got closer. The residents of Indalezio Ojanguren Street headed home, joining the parade that went through the streets, up the stairs and the escalators to the ground. “Imagine if on top of everything else, we actually win …” they said. Imagine. The debut was about to get even better. “Not even if our wildest fantasies,” ran the headline in El Diario Vasco. Last season, Javi Lara scored against Eibar. Now, with a minute to go before half-time, he scored the first goal Eibar had ever scored in the top division, the goal that would give them a first ever victory.

It was a wonderful goal too: a free kick from the left wing, hit right-footed, swinging and dipping over the goalkeeper Eñaut Zubikarai. Ipurúa leapt into the air. One fan who’d had a cigar and a skin full clambered onto his seat, shouting. “El Eibar es cojonudo; como el Eibar no hay ninguno.” [Eibar are the bollocks; there’s no one like Eibar.] Cojonudo is the word. No one could say it wasn’t deserved: Eibar had been the better team. By the full-time whistle, Real Sociedad had created chances, but the shot count favoured the debutants. “They were better than us,” admitted the midfielder Markel Bergara.

“We knew where we were coming and we knew who we were facing. I saw the same Eibar as always,” the Real Sociedad manager, Jagoba Arrasate, added. Intense, chasing every ball, never letting their opponents settle. Bold and brave, refusing to be cowed. “I told them not to be scared, to be themselves,” Garitano revealed. “You can’t crap yourself.”

Under the stand, you could smell the deep heat from the dressing rooms. Fading photographs covered the walls. Eibar in 1940, the year the club was founded. The forerunners: UD Eibaresa in 1927, CD Gallo in 1934. Every Eibar team picture since 1947. A photo too of one of the region’s traditions: the world record beech tree axe-chopping. One hour and 13 minutes to hack through three and a half metres of wood. Behind a scrum of cameras, the midfielder Jon Errasti, impeccable throughout the game, was about to give his debut interview as a top flight player, smiling and patiently waiting to go on air. “This is the first of 38 battles,” he said.

And it will be a battle. Eibar have no debt – the share issue means there is in fact a small surplus – but the struggle continues. Promotion costs. The minimum salary in the Segunda División is €60,000 a year; in La Liga it is twice that. Eibar will not go much beyond that, there’s no money for signings and there is little Primera División experience. Loans continue to be central to their approach. Eleven players went in the summer; nine have come in so far and there may be more yet. Officially they have only 21 players. Some of the new signings are not ready: Derek Boateng and Federico Piovaccari were left out on Sunday. “They don’t even know their team-mates’ names,” Garitano said.

But Eibar have overcome greater obstacles and on Sunday none of that mattered. Not yet. It was dark now but the night, like the season, had only just begun. “The players have worked so hard for this. I’m proud to coach them,” Garitano said. “What have I told them? I’ve told them to enjoy this.” Out of the ground, back down the hillside, past the names on the side of Ipurúa, and back into Plaza Unzaga, Eibar’s fans were doing exactly that.

Talking points

• It might just have been the best header you have ever seen. Gorka Iraizoz, who once played on loan at Eibar by the way, leapt high, his back straight, and thumped a header into the net from not far inside the area. It flew into the net and he ran off celebrating. He had just scored the equaliser for Athletic Bilbao in the 94th minute. He is also a goalkeeper. The first goalkeeper to score in La Liga since Dani Aranzubia. The referee Mateu Lahoz had different ideas though. He disallowed the goal for … well, erm, no one’s really sure. He gave one reason and the linesman gave another, Neither convinced. After the game, Iraizoz and Lahoz met again in the mixed zone and the debate went on. Eventually it finished with Gorka shrugging and saying: “The worst thing is it was a bloody good goal.”

• Sergio García scored the late equaliser as Espanyol got a 1-1 draw against Almería. And “late” is the word. It came in the 103rd minute after a power failure. It also came at 1.01 in the morning.

• Leo Messi scored twice, but for once he might not be the focus of attention. Or not the only one, anyway. The 18-year-old Munir El Haddadi got the other, and it was beautifully taken.

• Sevilla: good(ish) against XI, bad against ten. Lucas Orban got Valencia’s equaliser in the 88th minute, with his chest. It’s not the first time it has happened to Unai Emery.

• Time for another historic night. Córdoba travel to the Bernabéu on Monday, back in the top division for the first time in 42 years.

Results Málaga 1-0 Athletic, Sevilla 1-1 Valencia, Granada 2-1 Deportivo, Almería 1- 1 Espanyol, Eibar 1-0 Real Sociedad, Barcelona 3-0 Elche, Celta 3-1 Getafe, Levante 0-2, Villarreal Monday Real Madrid-Córdoba, Rayo v Atlético.