Euphorbia species- Flower Show, Mumbai
Cactus for ID : Flower Show,Mumbai : 210213 : AK-4: Another cactus seen at the flower show in Mumbai during Feb,2013.
Again a cultivated, potted, ornamental plant.

could it be the ORGAN PIPE CACTUS?

USA has national monument in its name...
see attached
screen shot of google search..

euphorbia sps
Yes ... This is Euphorbia species
Difference between Euphorbia and Cactus are as follows
The spines in a cactus are many and come from a circular area called aerole.
In Euphorbia there is no aerole and spines are only 2[modified stipules]

Euphorbia has white latex, cactus generally has transparent sap. All cacti are exotic, they come from America. Many Euphorbia species are indigenous [eg E.nerifolia, E.lactea.....]

In catus the flower is solitary [usually big and showy].

In euphobia it is a group of tiny flowers.. flower head.. male and female separate lacking petals [Chyme inflorescence]
Euphorbiaceae Fortnight : For ID : BBC Show,Mumbai : 061113 : AK-9 : Attachments (2). 4 posts by 2 authors. 
Seen at the BBC Show in Mumbai.
Suggested id was Euphorbia species on our group when posted earlier by me.
Kindly correct me if wrong.
This seems to be an introduced garden plant. I have not been able to identify it.
On searching, could this be Euphorbia mammillaris, Corn Cob Euphorbia?
Just a guess.