The short answer is nobody. However, as the question is completely made up with no bearing on reality its a little more complex than that. Sure, host countries provide the bulk of security to all organizations, meetings, and assemblies they hold be it private, governmental, civilian, and even international. However, in your scenario, as bogus as it is, your saying this evil dictator rose to power instantly and creates an instant police force that instantly terrorizes this poor international body. In this case, no nothing can be done.
In a lovely planet, I like to call earth, there are more factors at work. For example foreign presidents, ambassadors, diplomats, and all around big-wigs usually have their own private security detail, security from the country they are visiting, and security forces from their home country.
Evil dictators don't just pop out of nowhere. Usually it is easily visible when a host country is turning over to unfriendly elements. It may not seem this way to civilians living in said country, or to run-of-the-mill Joe Six packs around the world, but to those who follow the news and ESPECIALLY to foreign envoys it is pretty predictable. This does not mean down to the year or decade (unless they are also psychics like Cleo), but for sure down to the week.
I will give you an extreme example: Cambodia during the Khmer Rouge. The Khmer Rouge were an ultra-extremist communist guerrilla army/political party in Cambodia during the Vietnam war. Nobody knew much about them or when they exactly would come to power. Because of the war in Vietnam movement was extremely limited in the country side of Cambodia. However, at a time when all Cambodians were totally shocked and all cities and towns were under an emergency evacuation form the Khmer Rouge, almost all diplomats/ambassadors made it out alive with the exception of 1 I think... one from the USSR and this is only because they stayed past the warnings given by the Khmer Rouge because they were assumed to be on friendly terms. What this example illustrates is that evil dictators usually when ascending to power are not trying to start wars with foreign countries. Ambassadors, presidents etc. etc. get advanced warnings, high security detail, and also their host countries fly army choppers to rescue them as in the case with Cambodia. I would recommend several books mostly by Ben Kiernan. To tell you the truth you strike me more as a movie guy. So here's a great one: the Killing Fields. Its a 70's style indie/low budget film that won tons of awards. Its got a decent amount of cool war scenes/action so even a non-reader like yourself can stomach it, and it even stars Jon Malkovich before he started "being" John Malkovich and just was.
So what can we learn from all this drivel? Its good to be on top no matter what the government you live under is or where you are in the world, even behind unfriendly enemy lines of made up dictators. Its bad to be on the bottom no matter where you are... even in your own home like when Tom Arnold was married to Rosanne who was physically abusing him and he was addicted to inhalants to try and hide from his problems.... Good Luck!