I have constructed a hall for the purpose of meetings. But now there is too much of echoing within the hall. even without loud speakers. With loud speakers, it gets worse. How solve this problem of echoing

Excessive room echo is caused primarily by two things - hard surfaces and high ceilings. The worst materials are glass, marble and concrete block. Next to this is drywall. The more hard surfaces you have, the worse the echo will be. For example an entirely drywall room is much worse than one with drywall walls and an acoustic tile ceiling. The sound simply bounces from the back wall, to the front, to the ceiling, to the floor, back to the walls, floor, ceiling and so on. The solution of course is to reduce the amount of hard surface area.You need to coat the wall with some sort of acoustical wall treatment. Carpet on the walls will work or you can purchase materials for that purpose. How much to install and how much area to cover will require experimentation. There are many choices and this is just one.
http://www.acousticalsolutions.com/
An online search will provide many options. Search will the term "Acoustical Wall Treatment"