I have two ISPs connected to different ethernet ports of the router. Along with the default route every ISP assigns a bunch of additional static routes via DHCP. How can I put routes gotten from the different ISPs via DHCP into different routing tables, i.e. give them different routing marks?

This can be done by using different values for default-route-distance, that actually applies to the distance of all dhcp routes. Then set-routing-mark in routing filter should be used.