Don't play the wrong instrument here

It used to be said that if you wanted to discover someone's religion in Glasgow, you simply asked which football team they supported: the Protestant blue of Rangers or the Catholic green of Celtic. At one o'clock this afternoon the "Old Firm" Derby between the two sides kicks off at Parkhead in the east end of Glasgow - the start of what has been described as "ninety minutes of hate". Today's match has been given a special resonance by the recent appointment of Martin O'Neil, a Catholic Ulsterman, as Celtic manager, and by a summer of simmering sectarian tensions in Belfast (the Shankill and the Falls Roads often fall eerily quiet on Old Firm day).

It used to be said that if you wanted to discover someone's religion in Glasgow, you simply asked which football team they supported: the Protestant blue of Rangers or the Catholic green of Celtic. At one o'clock this afternoon the "Old Firm" Derby between the two sides kicks off at Parkhead in the east end of Glasgow - the start of what has been described as "ninety minutes of hate". Today's match has been given a special resonance by the recent appointment of Martin O'Neil, a Catholic Ulsterman, as Celtic manager, and by a summer of simmering sectarian tensions in Belfast (the Shankill and the Falls Roads often fall eerily quiet on Old Firm day).

There is nothing in world football to compare with the sectarian ferocity of the Glasgow Derby - not even the match between Barcelona and Real Madrid, in which the pride of Catalonia clashes with the hated Castilians from the Spanish capital. Last week, as I walked the streets of the "Barrowlands" in the east end of Glasgow, I was aware of a peculiar low-level tension, as if people were bracing themselves for the challenge ahead. Everyone I spoke to, every taxi driver I met, had an opinion about the Old Firm game, about the long years of struggle at Celtic (Rangers have been in the ascendance for more than a decade) and about a century of rivalry.

The east end has long been considered a Catholic stronghold - the impoverished grey towerblocks and decayed streets where many thousands of Irish immigrants settled in the second half of the 19th century (it was from within this community that Celtic was founded in 1888). And yet, intriguingly, the estates next to the Celtic stadium in Parkhead are Protestant enclaves, areas from which Rangers draw their support and from where members of the Orange Order set off to march every summer, in grotesque parody of more murderous rituals across the Irish Sea.

To walk these streets is to encounter the tensions of Belfast transplanted to Glasgow: pro-UVF and UFF graffiti blight the walls and bus-stops; the occasional Union flag hangs from a window, and the blue shirts of Rangers are worn with defiant pride, as they are in the pubs of the Shankill Road. Glasgow has been galvanised by change in the past decade. Slums have been cleared, smart restaurants and boutiques have arrived in Buchanan and Sauchiehall Streets. The arts have flourished, the university has expanded, new businesses have emerged to drive the engine of the local economy. But the areas around Parkhead, with their boarded-up shops and derelict properties, have somehow been forgotten in the rush to embrace a cosmopolitan cappuccino culture, in the desire to create a gentler, more welcoming image for the city. In such places, the old sectarian resentment festers.

Meanwhile, less than half a mile away from the Protestant enclaves, the Celtic squad is preparing at its Barrowfield Road training ground for the big game. It's an unexpectedly warm morning, and the players appear relaxed, teasing one another like young boys. Nearby, a group of canny, street-smart lads dressed in jeans and Celtic shirts look on through the prison-like bars. When I approach them, they say how much they "hate" Rangers, how much they hate the "Protestant bastards".

Anyone who has watched an Old Firm Derby understands that such hatred is real. Simon Kuper, in his fine book Football Against the Enemy, describes how one Old Firm clash, in 1975, provoked two attempted murders, two cleaver attacks, one axe attack, nine stabbings, and 35 common assaults. Nowadays, of course, football is calmer, the surveillance techniques and policing at matches immeasurably improved, but the potential for serious violence remains.

When Paul Gascoigne arrived in Scotland to play for Rangers in the mid-1990s, he made a drastic mistake in his first Old Firm match. Celebrating a goal, Gazza was encouraged by teammates to play an imaginary flute before the massed ranks of Celtic fans. Their fury shattered him. It was only later that Gascoigne understood that his act was a provocative symbol of Loyalist supremacy; that to play the "Orange" flute was to risk, if not death, then multiple death threats, as he soon discovered when, days later, a car drew up alongside his own at traffic lights. "This big guy wound down his window and called out to me," Gazza said. "He went, 'Gazza, you better watch what you're doing up here, man. If you ever play that flute again, we'll cut your throat'."

Celtic stadium, which has a capacity of 60,000, is the second largest in Britain. It is, like the larger Old Trafford, a magnificent theatre. As I sat in the Jock Stein stand last week, looking out across the empty terraces and down at the immaculate green turf, in thrall to the cathedral hush of my surroundings, I tried to picture what lay ahead - the sound and fury of today's game.

I recalled, too, how shocked Margaret Thatcher was said to have been when she was guest of honour at a Scottish Cup final in the 1980s and witnessed Celtic fans waving Irish republican tricolours, heard them jeering at the national anthem and singing pro-IRA songs. Poor old Mrs T - she had obviously never bothered to visit Celtic Park where the Irish and Scottish flags, but not the Union one, are raised proudly above the stadium. Nor had she listened to Rangers fans singing the Sash or mocking the dead IRA hunger striker Bobby Sands.

Later I hitch a lift back to the centre of town with an old chap who turns out to be the father of the former Scotland and Celtic defender Tosh McKinley. McKinley Senior is a terrific character: a passionate Celtic fan (he shows me the tattoo of a green and white hooped Celtic shirt on his left forearm) and a proud father. He recalls how Tosh once broke the nose of Henrik Larsson, Celtic's talented Swedish striker, in a training-ground brawl. "Tosh didnae want to hit him. But Larsson raised his arm first. If a man raises his arm in Glasgow, ya hit him. Ya hit him before he hits ya. Afterwards Tosh said, 'Didnee not know where he is, like, that he's in Glasgow now. You don't raise your arm in Glasgow and expect not to be hit.' "

There are many things, it seems, that you don't do in Glasgow, certainly not at this time of year - such as wearing the wrong football shirt in the wrong part of town, such as singing the wrong kind of songs or playing the wrong kind of instrument. Back inside Mr McKinley's car, the right kind of songs are playing: Irish folk songs, in which someone who sounds like Shane McGowan of the Pogues celebrates his love of Celtic and contempt for Rangers. "This one's great," says McKinley, leaning across to amplify the sound. "It's all about big Duncan Ferguson [the despised former Rangers striker, now at Everton]."

The car eventually stops and I jump out. I have journeyed only a couple of miles across town but it is as if I'm in another country. There are no football shirts to be seen and the crowds on Buchanan Street have a kind of metropolitan hauteur. Yet when the whistle blows this afternoon, I shall regret not being there to hear that Old Firm roar as 60,000 people once again spin deliriously into hatred.