I've found this plant growing in a crack in the wall. I then put it in a pot and is growing fast. Can anyone help me identify it, I live in Buenos Aires, Argentina.
This is how it looks now (2016-2-10):
I'm going to say I believe that is a Morus alba, white mulberry, which is becoming quite invasive in Argentina (and pretty much everywhere else it's been introduced).
It's hard to tell at this stage, but that's my best guess. If you want to let it grow out a bit, that might make an absolute identification apparent.