Madrid has prepared every last detail of today's royal wedding and primed its security forces against terrorist threats, but all the king's men are helpless in the face of forecasts of rain.

The nuptials between heir to the throne Prince Felipe and former television journalist Letizia Ortiz have obsessed and captivated a nation still wounded by the March 11 train bombings.

Felipe and Letizia will marry before 1,400 guests at Madrid's Almudena Cathedral and millions of television viewers worldwide, before parading through central Madrid in a meticulously planned festival of flowers and glamour.

The couple represent the next generation for a crown that has twice been deposed and restored and is credited with helping lead Spain to democracy after the death of dictator Francisco Franco in 1975.

Madrid has been dressed up for the occasion, and police yesterday began closing roads and metro stations along the parade route.

"It's the most difficult thing we have ever done at Madrid City Hall," Mayor Alberto Ruiz-Gallardon told reporters.

But he said it was welcome relief two months after the train bombings, which killed 191 people and wounded 1,900.

"Madrid absolutely needs this wedding," the mayor said. Security Secretary Antonio Camacho said police were prepared for just about any threat except the one from the weather, "given the forecasts that it might rain".

Johnny Mejia, an Ecuadorean who sells ice cream near the royal palace and cathedral, said police had been inspecting sewers, backpacks and unauthorised cars.

"Security has been very strict. They've taken our names because we will be here the day of the wedding," Mr Mejia said.

The towering prince - schooled as a diplomat, lawyer, soldier, sailor and airman - is from the Bourbon dynasty, whose reign dates from 1700.