It's worth pointing out at the start that all of our literary sources for the Scandinavian religions were written centuries after their conversion by Christian scribes, quite possibly taking creative liberties. Slavic religion is less well known than Norse because we have so few sources at all for it, but also because it doesn't undergo a neo-Pagan renaissance of interest, being eventually stamped out by the Holy Roman Emperor. One of our sources is Adam of Bremen's Gesta Hammaburgensis ecclesiae pontificum who writes in the mid-eleventh century about both the Norse and Slavic religions, but is fanciful in the extreme.
For the Norse, Adam records that there is a giant temple at Uppsala with a colossal gold statue of Thor, and vast sacrificial groves where human and animal sacrifices were carried out. The Wendish Slavs, he says, have a vast seven-walled, seven sided city where they worship a fiendish god called Radigast. At the same time, however, he blatantly rips off swathes of Pliny's Historia Naturalis to describe various monsters which supposedly live around the Baltic coasts, including Cynocephali and Amazons, so it's quite clear he's being very inventive here. For what its worth, extensive excavation at Uppsala have found no trace of any sacrificial groves, let alone any giant golden Thors.
A far more reasonable account of Slavic religion comes from Thietmar of Merseburg's Chronicon. Bishop of Merseburg, Thietmar compiles his Chronicon until shortly before his death in 1018, and unlike Adam of Bremen seems to base his writings on either his own experiences, or first hand witness testimony. In 1005 he gives an extensive description of Slavic religion so that "Although I shudder to say anything about them... in order that you, dear reader, may better understand the vain superstition and meaningless worship of this people. Thietmar is interesting as he takes pains to differentiate between the practices of the Redarii, the Liutizi and the Wends.
In Thietmar, Radigast is not a god, but Riedegost, a sacred shrine site in the lands of the Redarii:
A burg... which had three corners and three doors. It is surrounded everywhere by a great forest which the inhabitants hold to be inviolable and holy. Two of its doors offer entry to all. The third faces east and is the smallest. It opens on to a path leading to a lake...utterly dreadful in appearance.
In the burg, there is nothing other than a skilfully made wooden shrine supported on a foundation composed of the horns of different types of animals. Marvellous sculpted images of gods and goddesses adorn its outer walls... Inside stand gods made by human hands, each with a name inscribed and frightfully clothed with helmets and armour. Among them, Swarozyc occupies the first place and all the heathens honour and worship him above all the others. Their banners may never be removed from this place except in time of war and then only by warriors on foot
It's particularly interesting that although Thietmar condemns the practices of the Redarii, he can appreciate their arts and design, a curiously tolerant position for the period. The practices he details entail the casting of lots...
...when this is finished they cover the lots with green grass and, after placing two spears crosswise on the ground, humbly lead over them a horse... that which the casting of lots had already revealed to them should also be fortold by this almost divine beast.
Similar shrines, we are told, exist throughout the Slavic territories, which are decorated by war spoils and are occasionally the site of animal and human sacrifice, but Riedegost is the preeminent. Thietmar implies that several cults to Swarozyc are still active in the conquered territories of the Luitizi as a form of resistance to German, Christian conquest.