Is it completely illegal and do people who have committed it have to face fines and punishments?
I asked because the same matter has negative consequences in some other countries.
This depends on what you mean under "government" and under "against". A meeting against a current administration or President or certain government policies is legal, a meeting advocating the overthrow of the government, even if the meeting itself is peaceful would be illegal.
Whoever, with intent to cause the overthrow or destruction of any such government, prints, publishes, edits, issues, circulates, sells, distributes, or publicly displays any written or printed matter advocating, advising, or teaching the duty, necessity, desirability, or propriety of overthrowing or destroying any government in the United States by force or violence, or attempts to do so; or
Whoever organizes or helps or attempts to organize any society, group, or assembly of persons who teach, advocate, or encourage the overthrow or destruction of any such government by force or violence; or becomes or is a member of, or affiliates with, any such society, group, or assembly of persons, knowing the purposes thereof— Shall be fined under this title or imprisoned not more than twenty years, or both, and shall be ineligible for employment by the United States or any department or agency thereof, for the five years next following his conviction.
This is from US Code, title 18, § 2385.
If you organized an assembly or advocated the desirability of the overthrow of the US government, you could be subject to twenty years in prison.
Yes it is legal. It's protected by the First Amendment.
You can find such advertisements easily if you Google.
Here's a random hit:
(though, in all fairness, Occupy can't really be considered "against the government" given they are pretty much in sync with the current US Administration)