Day of the Beasts (1961) by John Glasby as John E. Muller

Earth was a crowded world of vast cities, manned by robots who carried out all of the menial tasks, who saw to it that everything contained functioning normally. Interplanetary travel was now an established fact. The planets favourable to Man's existence had been colonised but where, as yet, under-developed according to Earth standards.

In the whole of the Solar System, mankind was supreme. There was life on Mars, Venus and the outer moons of Jupiter and Saturn, but nothing which could match the military might of Earth.

Yet now, Earth itself faced destruction. Quite suddenly the thread had materialised. There was no doubt in anyone's mind that Someone - or Something - wanted Earth. But vast creatures such as these had never originated on any of the Solar Planets and Brad Norton, investigating events for the Military Commission, refused to believe that they could have been transported through space from an of the stars.

But the undeniable fact was that they were here and Earth science was powerless against them...