Explain the necessity of a constitution in the newly born democratic country

There are so many reasons a democracy needs a constitution.  A constitution sets up a nation of law, a nation the leaders of which cannot act arbitrarily.  For example, the United States Constitution ensures that American speech and religion will not be repressed by the government.  It ensures due process, for example the right to be informed of the government's charges against you and the right to a public trial. The government cannot act against you, in other words, without following the rules that the constitution guarantees.   A constitution sets up the duties of government and the relationship amongst its branches, so that one branch of government cannot arbitrarily seize power from another branch of government and so that each branch of government knows what its responsibilities and limitations are.  It sets up a means of peaceful succession of leadership, chosen by the people, as opposed to a coup or hereditary succession of power.  Without a constitution, a democracy can easily devolve into a far less desirable form of government, often dictatorship.  If the people elect a leader who is not limited by a written constitution, there is nothing to stop the leader from deciding to no longer hold elections and to stay in office for life.  There is nothing to stop leaders from acting in ways that are inconsistent with democracy in general.  For all of these reasons and many more, a democracy without a constitution cannot be a viable democracy.