I read this book while on vacation in Canada around 1985. The book seemed to be a recent release and consist of about 50/50 narrative and character action and was short, about 250 pages or so. The cover featured a menacing shadow of creature or creatures with slanted glowing 'eyes' (maybe fangs?) over an Earthly landscape of some sort, but I want to say pyramids and palm trees - it was a simple illustration. I realize a similar book from the time was Terry Brooks' 2nd Shannara novel with demons invading through portals; it is not the same book.
The book starts out almost like a horror novel with a few grisly murders and encounter with a vicious, fanged nocturnal creature. If I recall correctly, the opening chapters feature a humanoid who traveled through a portal is trying to make contact with government officials while evading these creatures which only seem to be bent on killing. At some point in the story it is revealed that the creatures accidentally leaked through while the alternate Earth humans were trying to seek asylum and escape their own overrun Earth. I can't recall if the alternate Earth was from Earth's future or just an alternate timeline.
Eventually contact is made while the whole world is on red alert fighting off the invading creatures. The alt-earthlings, in exchange for asylum, agree to pay the governments of the world in huge amounts of valuables dumped via portal in massive quantities. In one country thousands of tons of diamonds are dumped but I most remember a vignette scene where thousands of tons of rice are dumped in some impoverished African nation. A starving child witnesses the dump of rice and starts shoving handfuls of uncooked rice in his mouth only to vomit it up moments later because of his extreme starvation.
That is all I can remember - anyone remember this one from 30 years ago?