For decades, Mexicans from the arid mountains of Baja California to the lush hills of Chiapas have taken to the streets on November 20 to celebrate the revolution that toppled Porfirio Diaz's dictatorship in 1910.

But the tens - perhaps hundreds - of thousands of people who gather today in the Zócalo, Mexico City's main square, will be thinking of another kind of revolution: that proposed by Andrés Manuel López Obrador, their charismatic leftwing leader.