If it's specifically for a US readership, Ray's suggestion of "administration" for "gobierno" would be particularly suitable. For general purposes "government" would be fine.

I like "hotbed" here. It has two meanings:
"1. a bed of soil enclosed in glass, heated especially by fermenting manure, and used for forcing or for raising seedlings
2. an environment that favors rapid growth or development <a hotbed of activity>"
Usage example: "a hotbed of political unrest"
http://www.merriam-webster.com/dictionary/hotbed

This seems to me ideal. It's a very standard term for political unrest or agitation, it retains the reference to heat, and it reproduces the metaphor of heat making things grow.

For "surgidas" there are many possibilities. "Sprang up" is one, but you could also go for something like "sprouted", referring more specifically to plants growing, to underline the metaphor.

"The hotbed of criticism" is a phrase with a pedigree; Sydney Smith used it in 1823:

"we have at least one author at present, whose productions spring up free and numberless, in the very hotbed of criticism"
http://books.google.es/books?id=fC8eAQAAIAAJ&pg=PA349&lpg=PA...

--------------------------------------------------
Note added at 1 hr (2013-06-14 08:45:33 GMT)
--------------------------------------------------

In context I think "which had sprung up" would be better; it's talking about radicals joining forces with parties which had already emerged at this point.

As I say, I think the idea of "al calor de" is that the criticism is like the heat which makes seeds germinate.