Creating a concrete mix with smooth texture
Isherwood's statement is correct in that basement floors are smooth but the surface is made smooth using a power trowel. A power trowel would be much too large to use for your application. 

I suggest using an aggregate free Portland cement and sand (or silica powder if its available). The ratio is typically 1 part cement to 3 parts sand and water of course. The trick to getting a smooth final surface starts after its poured and before it sets. You will need to create or purchase a float that will be used to produce the smooth finish. After you've poured your mix into the mold, level it out and make sure no air pockets are below the surface. 

Within a few minutes the setting process begins. This is when you will see a layer of water develop on the surface. Using your float, skim the surface in one direction until the desired smoothness is achieved. It's that simple! 

Also, I suggest reinforcing your tabletop with steel. Chicken wire (as it's called around here) should work well or some other mesh type steel. Good luck and post some photos of your final creation!