Barcelona and Manchester are like two kissing Continental cousins. They are both proud regional centres with an impressive irreverence towards their respective national capitals; both to the left of the political spectrum; and they both love football. Barcelona's Poblenou district was once even known as "the Catalan Manchester", thanks to its textile industries.

They are both music cities, too. OK, Barcelona might not have a lineage of classic pop acts to compare to The Smiths, New Order et al., but it does have a history of cherishing the left field and avant-garde, thanks to festivals like Primavera Sound and Sónar. There's even a Manchester bar in Barcelona, which borrows heavily from the Factory Records aesthetic and delights in blasting out Manchester's indie hits.

The Manchester/Barcelona connection - as well as the Catalan capital's love of the avant-garde - was in evidence at this year's Barcelona Acció Musical, one of Barcelona's more intriguing musical events, albeit one that is largely unknown outside of Catalonia.

Barcelona Acció Musical - or BAM - takes place as part of the annual celebrations around La Mercè, co-patroness of the city. The festival has been celebrated since 1687, when the local council prayed for Mercè's help to free Barcelona from locusts. It features a lot of Catalan tradition, including castells (hugely impressive human towers, to which quivering children are sent to ascend) and the sardana, a circle dance noted mainly for the torturous lengths that participants have to keep their arms raised.

Since 1993 and the introduction of BAM La Mercè has also had a platform for modern music. BAM takes place in outdoor venues largely in the city centre, with minimal concern for noise regulations or pandering to the crowd. And it is all for free. In 2011 I witnessed King Midas Sound rattle the windows of the MACBA art gallery, just five minutes from the Rambla tourist trap, with their extraordinary bass pressure, a moment that made me realise that festivals in Barcelona are slightly different from those in Norwich.

The 2015 edition kicks off on Friday September 18 - six days before the actual public holiday of La Mercè - with Catalan teen quartet Mourn, who play to a large audience in Plaça Joan Coromines, a dusty town square that used to host Sónar by Day. Mourn represent that rarest of phenomena: a Catalan rock band generating global interest and from this performance you can see why. Their sound may be a virtual parody of early 90s indie rock - riffs straight out of the Babes In Toyland playbook, PJ Harvey-esque vocals and a sprinkling of Sonic Youth guitar experimentation - but they do it with such youthful verve that it reminds you that that grunge wan't just about heroin addiction and suicide.

It helps that duel vocalists Jazz Rodríguez Bueno and Carla Pérez Vas have fantastic rock voices, capable of deep bassy rumbles and throat-shredding screams, while the impossibly young rhythm section drives the music along in a tightly-wrapped bundle of power. They write great songs, too: new single 'Gertrudis, Get Through This!' is two and a half minutes of sheer teen ebullience, while Silver Gold channels Rid Of Me era PJ Harvey via the darker corners of The Pixies. Mourn's recorded output to date might still be a little raw but it is hard not to love them live.

After a band so clearly indebted to the early 90s, Loop - an actual band from the early 90s - should be the perfect follow up. And yet it doesn't quite work for them. They play in Plaça dels Àngels, a two-minute lurch from Joan Coromines, and the large crowd who has watched Mourn ambles over to check them out. Maybe it is just the contrast with the teen energy Mourn have expanded but there is something slightly weary about Loop's heavy psychedelia. Sure, there are some great rolling guitar riffs that fill the square with drone but the vocals are rather directionless and I find myself idly admiring the glassy beauty of the MACBA building, never a good sign at a gig.

From there it is a 15-minute walk down the Rambla to Moll de la Fusta, where the third BAM stage is found next to the Marina. It is a beautiful place to listen to music, lined by palm trees with a salty breeze coming off the sea, and the line up on Friday night reflects this tropical feel, featuring Jamaican blues singer Brushy One String and Ghanaian highlife guitarist / band leader Ebo Taylor. I only manage to catch 20 minutes of the latter but his guitar-led afrofunk is irresistible in the warm night air, as yachts bob rhythmically in the background.

Saturday is the main day for BAM, with 19 acts playing over five venues, including US folk-rockers Vetiver at Plaça dels Àngels and Cambodian pop revivalists Dengue Fever at Moll de la Fusta. But we head to El Parc del Fòrum, the weird exhibition space on the outskirts of town that is probably best known as the site for Primavera Sound.

The Forùm is empty when we get there at 10pm - early by Barcelona time - which means Shopping's Bloc Party revival and Barcelona garage rock act Nueva Vulcano play to relatively small crowds. Things fill up considerably for Tony Allen, though, as befits a man of his legendary status, once drummer and bandleader for Fela Kuti, now solo act and serial Damon Albarn collaborator.

Frankly, Allen could sit up there all alone knocking out his intricate polyrhythmic beats, his sedate face giving no indication of the furious funk his limbs are dispensing, and I'd be more than happy to watch him, such is the elegant majesty of his drumming. Every song he plays is a masterpiece of rhythmical interplay, the beats never falling quite where you expect them, as if the standard four four funk template is something for lesser mortals to observe.

His fantastic band, who add melodic flesh to his rhythmical skeleton, are the icing on the cake. The bass player in particular shows incredible understanding with the drum legend, his jazzy, rock-solid playing serving as both anchor and springboard for Allen's drumming runs, while the two-piece brass section gives a taste of the classic Afrobeat horn riffs. Even the additional percussionist - not a job for the faint hearted, you'd imagine - has his moments, producing what is the funkiest five minutes I have heard from a triangle in the show's closing cop-show funk pastiche. As we leave, Allen-ed out at 1.30am, crowds are streaming into the Fòrum. They have Delorean to look forward to. But they don't know what they've missed.

Sunday is a day of relative calm for BAM after the exuberances of the previous night (the Fòrum closed with a back-to-back set from local DJ heroes Pau Roca and Sau Poler, which kicked of at 230am). And what better to ease our buzzing heads than the soothing post-FKA Twigs alt-pop of Roseau, drafted in to replace the similarly minded Georgia at the last minute? She's up against strong non-musical opposition though: there's a correfoc - a horde of locals dressed up as devils setting off fireworks in the street - rampaging through central Barcelona and Barça are also playing at home, which explains the modest crowd that greets Roseau's first song, 'Salt'.

She eases slightly nervously into it, her voice exposed by the song's minimal production, all spartan electronic beats and skeletal keyboard lines, which echo round the cavernous Plaça dels Àngels. It's a sleepy, tentative start but things pick up with 'Hot Box' and 'Alright', both of which prove Roseau can write a stirring chorus, and by the time the relatively upbeat Florida arrives the singer is in great voice and there's been an outbreak of public snogging. Roseau ends the gig with Accelerate, replete with bewitching, twangy guitar riffs and strident drums, having decisively won over the crowd. "This has been fucking amazing," she says by way of goodbye, which might be overstating things a little. But it has been a promising introduction to a budding talent.

Tuesday is the night that Manchester comes to town (or the Plaça dels Àngels, at least) courtesy of Lonelady, possibly Manchester's most emblematic act of the moment. She's even brought some Manchester weather with her, Barcelona turning ominously chilly for the first time since March. Luckily, though, Julie Ann Campbell plus three-man band bring enough steely city disco to warm up the coldest crowd.

Their live sound, much as their records, is a kind of taught, minimal, funk, that builds to never quite release, and the band is on driving form through a strong opening duo of 'Into The Cave' and 'Bunkerpop', mirroring the opening one-two of the recent Hinterland album. The latter song in particular is fantastic, with jazzy chord changes ratcheting up the pressure.

Things dip slightly during a middle section that is slightly one-paced - although what an enjoyable pace it is, like Joy Division meets LCD Soundsystem - but the closing sequence of 'Groove It Out' and an extended 'Hinterland' utterly wins over the Barcelona crowd, appealing to the kind of Catalan indie kids who worship Joy Division and made Pulp's 'Common People' into a Barcelona anthem. 'Hinterland', in fact, may be the highlight of the whole gig, transformed from a warmish pop song on record into an epic gig closer, that rides along on a steely violin riff. The crowd frugs along with gusto, as a new Barcelona - Manchester romance is cemented.

Will Manchester love Barcelona right back? Certainly, BAM would be a great place to kindle the spark: it's not just six days of brilliant, genre-stretching music, it's also a festival that lifts the lid on life in modern Barcelona. It is a festival, in other words, both in and of the city. BAM, like Barcelona itself, respects tradition but gives space to new ideas; it is international, youthful and future facing; and it likes to let its hair down on a Saturday night. What city could resist?