Tomorrow is a day the people of Derry have waited a long, long time for: that of the official opening of the Millennium Forum. A spectacular event is in store for the largely invited audience of the city's great and the good, as well as those who have laboured long and patiently for the facility.

It chief executive, Michael Poynor, whose reputation for creating lavish, large-scale musicals and Christmas shows goes before him, will be the animateur, doing his usual breathless thing of designing, lighting, choreographing and directing local performers in a theatrical showcase whose star, he insists, will be the building.

The idea of a theatre for Derry has been around for 30 tortuous years, going through the hands of planners, architects, city fathers and funding bodies. Poynor recalls being shown a model of the proposed theatre in 1973 by Frank Murphy, who was then drama director of the Arts Council of Northern Ireland.

Anne Craig, formerly a producer in BBC Northern Ireland's Irish-language unit and now the Millennium Forum's education officer, remembers buying a brick for a new theatre as part of a fund-raising exercise in 1974. Like many of her contemporaries, she was involved in the early days of Derry's Theatre Club and the 71 Players, and can hardly believe the dream has come true.

"When I was growing up, theatre was a major part of the social life of Derry," she says. "We didn't even think of it as theatre. We just talked about 'putting on plays' and 'going to plays'. We were all amateurs, of course; what we were doing was a labour of love, totally unfunded.

"The membership crossed all age groups, as well as other social and cultural divisions. We rehearsed all over town, wherever there was a space big enough, and our group performed in the little hall beside St Colum's Hall."

Even back then, people were talking about the need for a proper theatre. Among those involved in the early movement were Brian Friel and Siobhβn McKenna. But with so many other things going on in Derry, it was put on the back burner.

Even in the early 1980s, when Field Day was set up as a theatre of ideas, plays such as Translations, by Friel, and Pentecost, by Stewart Parker, were staged not in a proper performance space but in the Guildhall, which had been converted for the occasion. "There is a tremendous sense of pride in the city that this building is here at last, for we had all begun to think we would never see it," says Craig.

Inevitably, there have been a few dissenting voices. A small group of local actors wrote to a local newspaper, calling the new building "a magnificent folly that fails to reflect or attract local talent". Also, in a recent interview on BBC Radio Foyle, Pauline Ross, the director and founder of the Playhouse community arts centre, gallery and dance studio, voiced doughty support for the new £13.5 million, state-of-the-art venue, while sounding understandably rueful at the stark contrast it makes with her more modestly funded operation.

Still, the integrity of the Playhouse, which Ross almost single-handedly resurrected from the run-down shell of a former convent, remains unbroken, providing the space this week for the Irish premiere of Midden, by the Derry writer Morna Regan, with which Rough Magic theatre company won a Fringe First award in Edinburgh last month.

Then there are those who feel the theatre's site is cramped, oddly shaped and lacking in dignity and presence. They point to the beautiful places along the River Foyle where a more aesthetically pleasing showpiece could have been built.

The counterpoint to their argument is that its position, slotted into the ancient walls, absorbs the Millennium Forum into the fabric of the city, right on the main shopping and walking routes. And while its high, wedge-shaped glass walls and slightly kitsch sandstone turret, bearing the venue's name in aquamarine letters, may not appeal to all tastes, you cannot deny the ingenuity of a design that shoehorned such a large-scale facility into such a tight, unaccommodating space.

Poynor is only mildly irritated by the charges. He points out that those who object on aesthetic grounds haven't come to look at the building, and adds that "in terms of local talent, our intentions could not be clearer. No less than 30 local events or groups have been programmed for the first year, most of them for the autumn season. And I am using 100 local performers in our all-day opening party, when there will be performances throughout the building: in the piazza, on the stage and in every available working space, including the box office, the office and the dressing rooms. The aim is to allow our invited guests to see just how versatile, how flexible, how vibrant and friendly a place the Millennium Forum is.

"I hear people talking about a '1,000-seat theatre in Derry' and voicing doubts about how it will be filled. But it is far from being only that.

"It's uniquely designed with a moving ceiling in the auditorium, which can accommodate concert-style performances of over 1,100, a medium-scale theatre with 715 seats and a more intimate 367-seater space. In addition, we have a small studio theatre and rehearsal room.

"The design concept works in everyone's favour. Derry has, in effect, three new theatres on its doorstep, offering a vast range of events and performances, and I am not faced with the near-impossible task of having to fill a 1,000-seat auditorium all the time."

After the first public performance, by the Ulster Youth Orchestra, during which some acoustic and sight-line difficulties came to light, epic efforts are being made to fine-tune the building, a task that will not be complete until the food and retail outlets that will help to pay its way are in place around Christmas.

It was this philosophy of creating a living, working, moving space, open and available to the people of Derry all day, all year round, that formed the basis of Derry Theatre Trust's successful bid to the Millennium Commission.

Poynor says the commission had declared itself uninterested in funding simply a theatre, but was impressed by the trust's community and integrationist approach. It is a line he has no difficulty in following.

"In my view, the Millennium Forum is exactly that: a meeting place, a forum for people to shop, eat, socialise, entertain, create and enjoy life together. From that point of view, it is in exactly the right place.

"If I had been offered a choice when it was in the planning stage, I might have said, yes, please put me on the river. But look at a beautiful building like the Waterfront Hall in Belfast, which lies outside the mainstream of city life and is like a tomb during the day.

"These are very exciting times for Derry. In and around the city centre we have venues like the Nerve Centre, the Verbal Arts Centre, St Colum's and, of course, the Playhouse, where Pauline Ross has done a fantastic job over the years.

"It is understandable that she should feel apprehensive about the arrival of the Millennium Forum, but I am confident that what has happened in Belfast will happen here. The opening of the Waterfront did not take away from audiences at the Grand Opera House: quite the contrary. The Lyric is having problems at the moment, but for other reasons - and it will come back. And now the new Odyssey Arena is regularly selling out its 7,000 seats.

"I believe that people are learning to use their leisure time to their advantage and are starting to acquire a habit for going out to all kinds of events. The city can now offer an enormous range of entertainment, creative and cultural activity, from pub entertainment to cinema, theatre, dance, rock and pop music, choirs, opera and top-quality touring shows like Rent and Phantom: The Ballet.

"We are at the hub of what I would call a cultural trail around the city walls, and I have no doubt that the Derry community, who have waited so long for this exciting building, will come and support us."

Midden, by Morna Regan, opens at the Millennium Forum tomorrow; bookings at 048-71264426