"Remember to pace yourself!" My Spanish friends have seen foreign visitors fall by the wayside in previous years and they want me to last the night. We're in the middle of a huge crowd waiting for the stroke of midnight, when the festivities will begin. Right on cue, the darkness is suddenly and stunningly illuminated by 22,000 coloured light bulbs decorating a 50m-high gateway. Behind it, a fairground lights up section by section in a wave of illumination, revealing an enormous tented city to which the whole of Seville will decamp for the week.
This is the official opening of Seville's annual Feria de Abril. Just two weeks after the world-renowned parades of Semana Santa (Holy Week) come to an end on Easter Sunday, the atmosphere in the city is transformed from sombre devotion to hedonistic excess, as the locals strip off their penitents' robes and squeeze into their sexy flamenco gear, because the Feria de Abril is when they really get the party started.
What began in the mid-19th century as a cattle fair has developed over the years into a post-Lent party: a spectacular six-day extravaganza, when the entire city devotes itself to the pursuit of pleasure 24 hours a day.
As we make our way through the gate of lights into the fantasy city, swept along on a wave of exuberance, we hit sensory overload: before us are more than 1,000 striped casetas (party tents), endless strings of multicoloured paper lanterns and brilliant lights amid a cacophony of live flamenquito groups singing passionate Sevillanas (the local folk music) accompanied by guitars, castanets and wooden boxes for beating the rhythm.
I'm astonished that everyone seems to know how to dance flamenco-style dances (also called Sevillanas) and to see people of all ages dancing spontaneously in the street, joyfully and unselfconsciously: grandparents, couples, children, mothers and daughters, and girls together.
I join a circle watching a group of women dancing together and am wishing that I'd had a few lessons when – to my alarm – they urge me to give it a try. Like a typical Brit, I feel awkwardly unable to pull off the sexy, dramatic posturing that comes so easily to the Sevillanas, but the women are so enthusiastic, welcoming and keen to teach me that I abandon my inhibitions. Following their lead, with my arms above my head, I mimic picking an apple, eating it and throwing it away while simultaneously swaying my hips in a way that I hope is seductive, and affecting a proud and haughty demeanour. I know they're just humouring the inglesa, but the round of applause I get for my efforts leaves me elated, and makes me feel a part of the celebration rather than just a spectator.
Feeling more confident now, and wanting more of the "insider" experience, my friend and I hatch a plot to gatecrash one of the private casetas: we reckon the worst that could happen is that they point out our mistake and ask us to leave. Each caseta is open at the front, offering a brightly lit window into an intimate party scene, so it's easy to identify one that looks too busy for anyone to notice an incursion.
Wandering nonchalantly in as though we have gold-plated invitations, we find the inside gaudily decorated, with multicoloured tissue rosettes covering the ceiling, brightly painted wooden tables and chairs, and gilded mirrors and paintings.
We head straight for the bar but, to our embarrassment, the host rushes over to greet us. We confess and explain we just wanted to see what the tent was like inside and are rewarded with a warm invitation to stay and join their festivities. It's almost like a private party, different only in that the catering is commercial, with a pay bar, a menu and a waiter. Not many people are wealthy enough to cater for hundreds of friends and relatives, six days in a row, night and day – and it makes us feel less like spongers for dropping in.
It's a relief to get out of the melee for a while, take the weight off our high heels and try a rebujito – a mixture of manzanilla sherry and lemonade – much nicer than it sounds, and more alcoholic than you might expect.
My earlier dancing practice comes in handy as the price extracted for our entry is that we join in a rumba, which involves everyone forming a circle and each participant taking a turn to perform in the centre.
Although most of the casetas are privately owned by families, groups of friends and companies, there are also some large public ones, where anyone can go to buy food and drink. On the opening Monday night, it's traditional to sample the delicious pescaito frito, a mixture of fried fish, prawns, squid and baby octopus.
At 4am and several casetas later, I'm seriously flagging, but my new Spanish friends press on me a glass of caldo (chicken broth), a remedy that, they tell me, is guaranteed to settle my stomach and set me up for a few more hours' drinking.
Before you get to the queasy stage, though, it's worth making a point of visiting Calle del Infierno – "Hell Street", appropriately named since it's a huge funfair alongside the tented city. Wait in the queue for the big wheel and you are rewarded with dazzling views of the whole area of the Feria, with its blazing lights. Apart from that, the bumper cars, rollercoasters, and amusement arcades are best left to the teenagers.
Finally, at 6am, we've had our fill of the Feria and start to walk wearily home. But we're not finished yet: just as my stomach thinks the night of gluttony has come to an end, I discover that there's an obligatory stop-off for a breakfast of thick hot chocolate and churros (fried dough) and we join the groups of revellers at the Churrería los Especiales kiosk on the corner of Triana Bridge just as dawn breaks.
• This year's Feria de Abril takes place from 23–29 April. Hotel Fontecruz (+34 95 497 9009, fontecruzhotels.com, doubles from €189), has a spa with hammam and is a great place to recover
