TRNSMT @ Glasgow Green, 7 Jul

Glasgow Green has long played host to major outdoor concerts, so it was no surprise that promoters DF Concerts announced TRNSMT, their vowel-adverse new festival, would take place in its leafy environs. The firm could be forgiven for choosing the easy option after T in the Park faced troubled waters following its enforced move to Strathallan in 2015.

While Scotland's largest festival is shelved, temporarily at least, DF hopes TRNSMT can slide in as a ready-made replacement. But the two events could not be more different. While T could be a bit rough around the edges, it offered hundreds of artists performing a variety of styles across numerous stages. Few other festivals could offer Calvin Harris on one stage while My Bloody Valentine performed on another. TRNSMT may boast a city centre location and a VIP bar in a Victorian glasshouse, but its initial line-up looks thin compared to TITPs of old.

But while some pine for the glory days of Balado, many others attending the first day of TRNSMT are happy to look to the future – 9.25pm to be exact, when Radiohead are due on stage. It's no exaggeration to say the other artists booked today are acting as warm-ups for Thom Yorke and his band of less than merry men. Many of them, including such indie titans as Belle and Sebastian, cheerfully admit as much on stage.

The King Tut's stage – although greatly reduced in size – is one piece of T furniture to make the move to TRNSMT. It's here Be Charlotte is bringing the power of some serious alt-pop to a healthy mid-afternoon crowd. The songwriter/performer/future fashion icon from Dundee has released just a couple of singles to date but is already proving herself to be a star in the making. Machines That Breathe is a stand-out of a short but sweet set, and a solid gold pop classic minted for festival crowds.

Following Charlotte onto the Tut's stage are Saint Motel. The LA quartet prove a popular draw with their polished brand of FM radio indie-pop, but any interesting edges to their music have long ago been sanded down. When they finally drop their 2014 hit My Type – more an irritating advertising hook than a song – there's a predictable roar of delight. But not even the attention-seeking stage climbing antics of A/J Jackson can save this insipid collection of tunes.

By the time Belle and Sebastian take to the main stage the grey skies have cleared and sun beams down on the Green. "We were in Madrid last night and it was chucking it down," grins Stuart Murdoch, dressed in a dapper dark suit. It's often joked that B&S are the house band of Glasgow's west end, but they're the ideal early evening choice for a festival in the city's east. Their 70 minute set is a pleasing stroll through selected highlights of their illustrious 20-year career.

Judy and the Dream of Horses – which Murdoch announces he wrote after walking through the Green after a drunken night in the nearby 13th Note Cafe – receives a particularly enthusiastic response. Long-term fans will be pleased to hear the melancholy Seeing Other People get a rare airing, while 2006's Another Sunny Day is a perfect choice for a memorable summer's day performance.

With the other stages drawing to a close, every last punter on the Green squeezes around the TRNSMT main stage for tonight's headliners. Radiohead are no strangers to Scotland, but an outdoor show of this size by a band who still rank among the biggest grossing acts in the world feels like a rare treat. Judging by the variety of accents in the crowd, many have travelled from across the UK and further afield to be here. When the five-piece stroll on stage at exactly 9.30pm – joined once again by Portishead drummer Clive Deamer – a hushed awe descends. It's no secret the group will perform a two-and-a-half hour set, so conserving energy is perhaps wise. A roar finally emerges when Thom Yorke sings the opening line of Let Down, one of the several OK Computer tracks we'll be treated to tonight.

It's 20 years since that landmark album was released and Radiohead have clearly decided it's time to give it another push. While they would never do something as nakedly commercial as playing an anniversary tour, there's a knowing nod when Lucky strikes up as the second song. On a warm July evening by the Clyde, with Yorke singing 'It's gonna be a glorious day', it's difficult not to get caught up in the moment.

As great as that sounds, this is not a flawless show. The PA system and general sound level seems too low. This is not the fault of the band – but more likely to be a restriction on a festival taking place in a heavily built-up area. During more ambient numbers, the audible chatter of the crowd is in danger of drowning out some of the band's dynamic. It's not an issue if you're standing square in front of the stage – but many of those watching from the peripheries will later complain of a sense of quietness.

At 11pm on the dot, the band give a brief wave and stroll off after a captivating performance of 2+2=5. It's time for the first of two encores. After something of a midset lull, the crowd finds its voice for a massed rendition of No Surprises. The appreciative vibe extends to Lotus Flower, a highlight of 2011 album The King of Limbs. While Radiohead have never been a band that relies on anthems to carry a set, and would likely wince at the phrase 'fan favourites', the home straight sees the group wheel out not one, not two, but three big guns. There's a suitably epic performance of Paranoid Android, with Jonny Greenwood once again showing why he's considered one of the most talented individuals ever to plug in an electric guitar.

"This is a very old one," Yorke says before The Bends, a song that proves they can still headbang with the best of them. But the highlight of the evening – and the opening day of TRNSMT itself – is Karma Police. It's perhaps the best known Radiohead song of the lot, short of Creep, but its ability to make several thousand half-drunk punters reach for the high notes and belt out 'For a minute there...' is something to behold. For a band often dismissed by detractors as elitist, Radiohead have an incredible knack for bringing people together.