[Mobhack]

There is no such organisation. They trust/rely on the international treaties protecting embassies.

They do hire Group 4 and so on as the doormen to inspect passes etc, or local equivalents.

For uniformed people stomping around outside the gates, they rely on the host nation to provide army or police/paramilitary. Those are who are supposed to keep angry mobs of locals away - does not always work. (The local authority may also use the troops/paramilitaries hanging round outside so as to ensure that their own people don't go there either, like in say the old Soviet Union).

If the place is deemed too be dangerous then they usually pack up and leave the country, perhaps leaving an attaché in some less-hated nation's embassy. They will then return when the domestic situation is more secure.

In some more militant areas, there may be a "technical" support team inside, but that is on an ad-hoc basis. The technicals might be contractors, or (less than likely) some of THEM... But if it is that insecure there - see pack up and leave above.

There is no uniformed service presence like the US marines at their embassies.