Architecture: From out of the big blue yonder: A British architect has given Marseilles a heroic palace for local democracy. Michael Spens unveils Le Grand Bleu

The final inauguration of the pounds 100m Hotel du Departement des Bouches-du-Rhone, designed by Will Alsop, takes place this September in Marseilles. But this week saw the technical completion of this remarkable building, the headquarters and council chamber for Departement Treize, which covers the city of Marseilles and a large swathe of the surrounding countryside.

Since the award of the commission to Alsop & Stormer in August 1990, Marseilles has been at first sceptical, then incredulous, and finally enthused with its new blue acropolis at Saint-Just.

In winning the competition for its design, Alsop played a knowing hand, making the most of the role of 'ingenu' pitted gladiatorially against Goliath. In 1971 he had been runner up to Richard Rogers and Renzo Piano in the competition for the Pompidou Centre, Paris. In Marseilles, 19 years later, the final choice was between Alsop and Sir Norman Foster.

The design for Marseilles had passed through several stages before reaching the final selection round. Even then it existed as an opaque white model. But, by the final run-off against Foster, it went through a sudden metamorphosis, changing from white to blue. Alsop eventually settled on a deep blue known as Yves Klein International blue, a colour devised by the French painter.

Le Grand Bleu, as the building is known locally, is 10 minutes by Metro to the north-west of the 'Vieux Port', the historic and cultural heart of Marseilles. The complex stands astride the Metro line on a previously derelict site of tram-sheds and temporary car-parking. The site is surrounded to the south and east by dual-carriageways bringing commuters into the city centre.

Witnessing the structure rising from the foundations, locals first dubbed the building 'maison des Stroumphs' (House of the Smurffs), before acknowledging that it had style as well as curiosity value, and finally settling on 'Le Grand Bleu'.

Le Grand Bleu is located in one of the poorer quarters of Marseilles, surrounded for the most part by unpreposessing apartment blocks, industrial workshops and commercial tat. The separate parts of Alsop's building seem a direct response to these surroundings as well as to local climatic factors, principally the wind. Yet, such striking visual elements as the wind spoiler on the lower administration block, and the 'aerofoil' structure on its eastern counterpart, are as emblematic as they are necessary. The 'aerofoil' houses the presidential offices, as well as a spectacular cafe providing the best available views of harbour, docks and islands out to sea.

The two administrative blocks that form the core of the massive building enclose a massive atrium, which in turn houses an 'ovoid' information centre developed as a free-standing variant of Alsop's prize-winning Cardiff Visitors' Centre. In contrast to the ultramarine blue exterior of the building (embellished by Brian Clarke's panel design), the interior has the pale green colour of a Mediterranean inlet, serene and even therapeutic. This cool tone is achieved not by paint, but by the use of white-bodied glass panels, which reflect sunlight as green light. The play of colour and light in the interior of this heroic building must be experienced to be understood.

Le Grand Bleu is not just an administrative centre. The third major part of the building, the 'Deliberatif', or council chamber, is entirely different from the administrative offices flanking the atrium. Here is a dramatic Constructivist building and one clearly designed to draw attention to itself.

The long structure of the ovoid-shaped 'Deliberatif' tapers to follow the line of the road. While it reveals a simple symmetry in plan, it is a complex shape three-dimensionally. The design accommodates a degree of distortion while comprising a system of triangular roof-decking panels with variable gaps that correspond to the double curvature of the outer skin. These allow the debating chamber and ancillary spaces within literally to breathe. A long terrace runs the full length of the 'Deliberatif'; it is protected by sun-screens against the extremes of the midday sun.

Back in the atrium, there is a well balanced harmony of form enhanced by the modulated rhythm of columns: this conveys a gravitas which can only be defined as classical, in the sense of timeless. And these constitute no ordinary structure of support. They comprise 13 concrete columns engineered to form a massive lattice. Developed by Alsop, but refined in consultation with the engineers Ove Arup, these columns define the primary structure of the building.

These X-columns are a new departure, a hallmark of a building that kick-starts Modernism into the 21st century, as provocative as those Russian Constructivist fantasies of the Twenties. The impact of this departure from convention is best experienced as the visitor enters the atrium - a kind of internal Place de Ville - through sliding doors between the Xs. From this point, the whole building, despite its superficial complexity, is readily understood. An escalator leads up to the entry point for the 'Deliberatif'; lifts rise to the twin administration blocks. Here, one senses that Le Grand Bleu is, despite its radical appearance, a direct descendant of those highly embellished French town halls of the 19th century, when civic pride was at a premium.

Today, if European democracy is to succeed, the new regional centres of elected government are an essential counterbalance to the European Parliament. At Marseilles, regionalism is invested with a powerful significance in the very design of this heroic new building. In fact, the sheer visual power of the building means it is becoming known throughout the world.

Unwittingly, Alsop has also provided a building with an intriguing historical provenance. In 16th-century Florence, Vasari showed the city fathers how the widely dispersed functions of Florentine administration and taxation could be grouped together adjacent to the seat of the executives, the Palazzo Vecchio, in two parallel blocks or galleries. He then closed the river end, beguilingly, with a semi-transparent loggia. And he linked the buildings on to the Palazzo Vecchio by an elevated bridge.

Alsop's atrium at Marseilles offers the same very elongated two-point perspective and unwittingly, too, has also matched, maybe even trumped Vasari, with the ovoid 'Deliberatif' slung elegantly in at the southern end.

Today, the Uffizi art galleries occupy Vasari's spaces; who can not say that one day, too, Le Grand Bleu might fulfil a different role, no longer housing functionaries, but filled instead with works of art? Buildings today, says Alsop, must adapt and change.

Meanwhile, until the need to change occurs, one wants to agree with him that perhaps the appropriateness of form might ensure better political decisions will be made there. The 'Deliberatif' is the expression of that democratic function. Does form then swallow function?

Michael Spens is author of 'William Alsop: Le Grand Bleu', to be published by Academy in September, pounds 24.95.

Hotel du Department du Bouche du Rhone, an exhibition of paintings by Will Alsop, Mayor Gallery, 22a Cork Street, London W1 (071-734 3558), until 16 September.

(Photographs omitted)