Although it is not usual to attempt a "summary" of a poem (an analysis is usually what is desired by professors), I am happy to attempt one for you. First, let me direct you to my recent critical analysis in the link below as well as many other expert answers about Octavio Paz's poem. As you have most likely viewed the critical analysis, I would assume you prefer a simpler explanation which is why you requested a summary. What follows is my best attempt.
Octavio Paz begins with the words that he will repeat through the entire poem: "I speak ..." In this way, Paz makes the unnamed city both personal to him and also personal to the reader (in that Paz can be speaking of ANY city). This makes his poem universal.
Octavio Paz continues with vivid descriptions of his nameless city. This makes the city a living, breathing being for us. We can all imagine the sights, sounds and smells. (You will see he begins almost all aspects of this poem with the "I speak" moniker.) Note the following excerpt:
I speak of the markets with their pyramids of fruit, all of the flavors and colors, the smells, the tide of voices – water, metal, wood, clay – the bustle, the haggling, the conniving as old as time.
Not all the images in the poem are positive. Some are cold and unfeeling (as is usual when speaking of some aspects of a city). Paz now speaks of "the paralytic slum" and the "garbage heaps." He goes even further. Note the purposeful lack of feeling here:
I speak of the buildings of stone and marble, of cement, glass and steel.
Octavio Paz now continues with his idea that this city should be universal to all of his readers. We dream about this unnamed city. Perhaps even more importantly, the city dreams of US. Again, we see the city as a living and breathing character.
The city that dreams us all, that all of us build and unbuild and rebuild as we dream,
The city we all dream, that restlessly changes while we dream it
Eventually we, as readers, are engrossed and enveloped by the city completely. Paz wants us to lose ourselves. The city is that powerful:
Is that music coming closer or receding, are those pale lights just lit or going out? Space is singing, time has vanished: it is the gasp, it is the glance that slips through the blank wall, it is the wall that stays silent, the wall.
We end, then, with the idea of the city's power. It is ruled by the "despotic memory" of the "stern ghosts" of the dead who built the city. We can choose to take the positive sensory experience or the more negative concentration of the city's power; however, we must admit that the city is supposed to overwhelm the reader.