I found them growing in the cracks of huge old walls in the historical part of town.
Some time ago I have been searching in the Asteraceae but got nowhere..
The yellowish flowers never seemed to really open wide.

I think the flowers are seed heads Myriam (but I doubt you meant they were flowers). It looks like Conyza Canadensis. the first leaves are bigger but when it gets towards the flowering part they become small.

https://www.google.co.uk/searc...

https://www.google.co.uk/searc...

Thanks Janet,
I meant the small buds which appear to be flowers and don't really open up like you would expect, unless these are the immature seed pods but still there are the tiny ones that would be flower-buds..I was a bit puzzled by this plant..

I remember now I came upon the pictures you linked too, but dropped it as the leaves on mine are much more narrow, linear, also on the lower levels of the stems..
But maybe that could be caused by the growing condition on a wall..?

Another pic that shows a whole plant, the lower leaves are linear too..

I've made a crop to show what must be flowers..
A petty I didn't have my macro lens, too risky to walk around with it in Rio!

Look at the photos of Conyza canadensis on FloraSBS Myriam, a full shot shows all narrow leaves. It might depend on growing conditions, or a quirk of the species.

https://sites.google.com/site/...

Another ..

http://wnmu.edu/academic/nspag...

Maybe it grows larger leaves in healthy soil.

There are other species of Conyza .. the flower bracts seem to be shorter on your plant.

Numerous capitula of Conyza canadensis with very visible white ligules (when compared to the other species of Conyza )

http://invasoras.pt/en/gallery...

https://www.google.co.uk/searc...

https://www.google.co.uk/searc...

https://www.google.co.uk/searc...

The flowers look more like Conyza bonariensis, the leaves I'm not sure about as they seem to vary.

http://www7a.biglobe.ne.jp/~fl...

Another, there's too many to choose from! Best to see which species are growing in the area.

https://www.google.co.uk/searc...

http://www.calflora.net/losang...

Erigeron bonariensis?

https://en.wikipedia.org/wiki/...

Conyza bonariensis is a synonym of Erigeron bonariensis ..

http://www.theplantlist.org/tp...

When I search for "Erigeron bonariensis rio de janeiro" I only get Conyza canadensis ..

https://www.google.co.uk/searc...

I am convinced Janet, it is Conyza canadensis, especially the one in your second link which even has the same narrow leaves..Amazing how much photo material of the plant there is on the web!

Thanks for your patience!

It does seem to be the only one it can be Myriam.

I had photos in our database of the plant from Belgium which is very abundant over here, I even had one sprouting in one of my plant pots last year!

Really strange I didn't recognize the plant, the flowers/ seedheads of the Belgian ones are much smaller and more abundant, look more 'weedy', the plants in Rio looked more like a perennial, more sturdy..How growing conditions can influence the looks of a plant!
Horseweed (Erigeron canadensis)

It appeared in my garden a few years ago too, it sets around a little too easy although I quite like it. Climate and soil conditions can make a big difference.