I have been to both cities and I agree, Avenida dos Aliados in Porto and Wenceslas Square in Prague seem to be look-alikes. I am supporting the claim with some photos:
I have found out other important facts about the Avenida dos Aliados, considered the most important public space in Porto. Is that was remodeled in 2006 by Alvaro Siza and Eduardo Souto de Moura, the renowned architects and Portans themselves. Besides its representative works of architecture, the central city avenue and square, built at the beginning of the 19th century, used to have vivid mosaic pavements, lush green ambiance, but also the problems of traffic chaos and impossibility of mass gatherings.
The keywords for the 2006 project were ordered traffic flow, opening new vistas to the works of architecture in the Avenida and in side streets, and creating a central space for gatherings. However, the outcomes were not so happily accepted, and it was particularly difficult for the citizens to accept the removal of the green spaces.
This is how the Avenida dos Aliados used to look before:
Some more info can be found at: http://monumentosdesaparecidos.blogspot.com/2012/05/jardins-da-avenida-dos-aliados-porto.html