Ten of the world's leading architects on Thursday detailed their plans to dramatically transform the French capital into a Grand Paris, in what has been described as the most complex city project ever.

Nicolas Sarkozy, the French president, asked the architects, including Britain's Richard Rogers, to project 20 years into the future and dream up the world's most sustainable post-Kyoto metropolis.

Among the more outlandish plans is Antoine Grumbach's proposal to extend the city all the way to the Channel port of Le Havre via Rouen along the Seine, maximising the green possibilities of the river. The idea was already mooted by Napoleon Bonaparte, who said: "Paris-Rouen-Le Havre: one single city with the Seine as its main road."

Christophe de Portzamparc, the prize-winning French architect, has proposed building four …