Constitutional changes planned Morales' plans to overhaul the constitution.
He says this will give more political power to the country's indigenous majority and tighten control on provincial government spending.
The president's land reform plan, which calls for redistribution of idle land to poor peasants, enjoys widespread support.
But many people in eastern Santa Cruz, the country's economic powerhouse, fear their lands could be confiscated.
"We invite the people from Santa Cruz and the Bolivian people to join [the hunger strike] in defense of democracy and the state of law," German Antelo, a protest leader, said in Santa Cruz.
Earlier this week thousands protested in the city against against land reform. The protesters also called for more autonomy from the central government.
New proposal
On Friday Morales triggered new anger when he proposed that the parliament's upper house - controlled by the opposition - should be abolished.
"Those that do not defend the poor or the majority are generally in the Senate," Morales said.
"Why do we need a Senate, where there's still a majority of neo-liberals who will boycott it? That is the best argument that the new Bolivia we seek should be unicameral."
Analysts say some of Morales' plans are fueling economic and racial tensions between the European-descended minority of the eastern regions and the indigenous majority that populate the Andean highlands.